Lime Plastering: Part 2
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Following on from last weeks post, Lime Plastering Part 1, today we’re going to look at the plastering process in more detail, the plaster’s we have used, and a brief look at paints and finishes. However, before we go any further, I just want to clarify that whilst I am a firm believer in the merits of lime plaster particularly for older properties, there are situations where gypsum plasters are the better solution. The ‘rules’ around lime plaster are not hard and fast, every property must be assessed individually and a decision made based on what’s best for that house, and for its owners. Whilst lime plastering may theoretically be the most sympathetic material for use in historic homes, we have to be mindful that it isn’t always the most accessible.
Lime plastering is a highly specialised skill, and whilst most plasterers are well versed in gypsum work, great Lime Plasterers tend to be few and far between and very, very much in demand. We know that at the Farm we are very fortunate to be able to use lime ourselves; we had the time to learn to plaster, the practical skills to be able to do it, and the finances to buy the materials. This isn’t the case for everyone, and that’s ok! If you’re employing a tradesperson to plaster on your behalf, lime plastering is a much more expensive process than gypsum, as it’s much more labour intensive. All of that to say, if you read this post and by the end decide that lime plaster isn’t for you, there’s no judgement here.
It’s your house, and only you can ultimately decide on the best course of action for both you and for your home.
As an example of this, and to illustrate that whilst we are lime enthusiasts we’re not militant ride-or-die, my-way-or-the-highway fanatics, is that we have encountered situations in the Cottage where lime hasn’t been the most appropriate choice. Whilst we have used lime on most of our external walls, we have left the existing gypsum in place in the Cloakroom and the Kitchen. Why? Well, firstly because neither of these walls exhibit particular signs of damp. Secondly, the cloakroom wall is approximately 1m wide x 2m tall, and has a window in it, meaning the area that is gypsum plastered equates to around 1m2, so it’s not particularly worth the work. And finally, because we honestly can’t afford to rip out a perfectly good, functional kitchen for the sake of replacing plaster that at the moment isn’t causing (visible) harm. Whilst we may rethink this in the future, particularly long term if we come to replacing the kitchen, for the moment it’s the right decision for us, and that makes it okay in my book! We also haven’t used lime on any internal walls in the Cottage, as our internal walls are either plasterboard or built from Aircrete blocks (similar to breeze block), which aren’t breathable materials and so the benefit of plastering them in lime would be negligible. In these settings, gypsum is easier and more appropriate to apply.
With that said, there are settings where, if you can do so, lime plastering will hugely benefit your home. We covered this in Part 1 of this post, so head over there if you want a refresher before reading on!
Preparing for Plastering
On to the good stuff! Before applying fresh lime plaster to walls, it’s important to ensure the walls are properly prepared. In order for lime plasters to perform at their best, the walls need to be able to breathe, and so any cement renders, mortars or plastic paints need to be removed ahead of application. Once the walls are prepared and any non-breathable materials have been removed, any previously damp walls need to be given time to dry out ahead of plaster being applied.
Base coat of Lime Hemp in the Bootroom
What’s in Lime Plaster?
The ingredients of any lime plaster or mortar will follow the same general recipe, but in different ratios for different finishes and applications.
Lime + Aggregate + Additives + Water
There are different types of lime and aggregates that are used in plasters depending on the final application. Additives can range from pozzolanic agents to aid the set of the plaster, to hair added for strength, or minerals added to colour the plaster. A more detailed breakdown of these elements can be found in Lime Plastering Part 1.
Lime Hemp Plaster
In the Bootroom of the Cottage, we made the decision to try Lime Hemp Plaster, which we sourced from Ty-Mawr Lime. It’s breathable and helps to regulate humidity, making it ideal for use in solid walled and timber framed buildings. If you’d like to learn more about breathability, have a read of my Dealing with Damp blog post.
What’s it made from?
Ty-Mawr’s Lime Hemp plaster is made from non-hydraulic lime (aka Lime Putty/Fat Lime/High Calcium Lime), blended with hemp fibres and a pozzolan to aid the set of the plaster. As Lime Hemp is made using non-hydraulic lime, it requires exposure to Carbon Dioxide in the presence of moisture to set.
Ty-Mawr’s Lime Hemp plaster is made from British materials, so if you are in the UK and are particularly mindful of the carbon footprint of your renovation, this could be a great option for you. The plaster uses the ‘shiv’ of the hemp plant, which is the woody stem, and is a natural and renewable material. The use of hemp as an aggregate, as opposed to traditional sand, means the plaster can be applied in 10-25mm coats, thicker than normal lime plasters, making it quicker to build depth in relatively few coats compared with standard lime plaster. It’s a particularly useful plaster for dubbing out (a process I’ll explain further in this post) or patching. Hemp fibres in the plaster adds strength and flexibility, as well as having thermally insulative properties and helping to improve airtightness, which can in turn improve the thermal efficiency of a property by reducing draughts. The high flexural strength of Lime Hemp plaster makes it very durable, so it perfect for use in heavy traffic areas, like the Cottage Bootroom will be.
How does it come?
Lime Hemp can be purchased in pre-mixed 20kg bags, which is ready to use and just requires knocking up using a cement mixer or paddle mixer. If you are feeling particularly brave you could mix by hand, but be aware this is a very physical process and quite hard work! If you prefer, or you’re going to be working with the Lime Hemp on a larger scale, you can purchase the different elements individually in bulk bags and combine them yourself. We chose to purchase the Lime Hemp premixed, to save time and storage.
What is it suitable for?
Lime Hemp plaster is suitable for use in base coats (dubbing out, scratch and float), and can be left as a textured finish coat or can be skimmed with a fine lime plaster to finish. It can be used externally as a base coat for render, but Ty-Mawr will be able to better advise on this.
Where are we using it?
We have only used the Lime Hemp in the Bootroom of the Cottage. There were a few reasons for this decision. For example, we didn’t use it in the Lounge as it isn’t suitable for use in damp environments. The Lounge walls are earth banked, built into a slight hill, and so have the potential to be persistently damp. If applied in damp environments, the Lime Hemp could potentially never truly dry out, and so could rot, which would cause further issues and perpetuate any damp issues.
Another reason we’ve chosen not to continue working with the Lime Hemp is that whilst it does have some thermal value (U value of 1.26w/m2k at 30mm thick), it’s not the most effective insulating option, though is more thermally insulating than standard lime plaster. We also found applying the Lime Hemp to be a particularly physical process, which, in my opinion, was far more strenuous than other plasters we have used. Whilst I am pleased with the finished effect the Lime Hemp has provided in the Bootroom, the drying process has taken far longer than other products we have used, and therefore we decided not to continue using Lime Hemp elsewhere in our renovation, other than for dubbing out.
How much do you need?
1x 20kg bag will cover approximately 0.6m2 at 25mm coat.
Cornerstone Insulating Render applied in the Cottage lounge.
Cornerstone Insulating Render
In almost every other room in the Cottage, we have used Cornerstone Insulating Render from Cornish Lime. The Insulating Render is a lightweight breathable plaster, with high vapour permeability, which has the added benefit of being fire retardant and acting as a dehumidifier.
What’s it made from?
Cornerstone Insulating Render is made in Cornwall from natural hydraulic lime, and will set when mixed with water. The strength of the lime (binder) in the Insulating Render is an NHL2, which offers high vapour permeability and creates a flexible finish which can help to remedy damp issues. The aggregates used in this render are recycled, and it doesn’t contain any organic content so will not rot. The recycled aggregates were a big incentive to us, as we are trying to be as mindful of our environmental impact with our renovations as we can be.
Rather than using a sand as an aggregate, as in normal lime plaster, the aggregates in Cornerstone’s Insulating Render have a three-dimensional structure, which helps to improve the durability and insulating properties of the render. The render also contains synthetic fibres which improves the flexural strength and negates the need to use mesh when applying. The specialist aggregate and addition of synthetic fibres means the plaster is more lightweight than standard lime plasters, and can be applied to 30mm depth in a single coat. Being able to apply thicker coats can significantly reduce labour time if you’re looking for deep coverage from your plaster.
How does it come?
Cornerstone Insulating Render comes premixed in 9kg paper sacks. As the binder is a natural hydraulic lime, it dries through a process called hydrolysis which can only take place when the lime mixes with water. Therefore, unlike non hydraulic lime plasters, the Insulating Render comes in powder form rather than putty.
What is it suitable for?
The Insulating Render acts as a plaster, insulator, and dehumidifier all in one. It’s highly flexible and vapour permeable, with advanced thermal performance, making it an ideal choice for solid walled masonry. It has been specially designed for application into solid walled and historic properties to help to reduce heat loss. It acts as a lightweight base coat, improving insulation and moisture management. As it uses St Astier NHL2 as a binder, it’s suitable for use on weaker walls such as those made from cob.
The aggregate used in the render is non-porous, meaning it helps to regulate moisture movement through the wall and reduce cold spots, which improved air quality by reducing condensation. It can be applied directly to the wall, so doesn’t require framing or boarding the wall before application, which is helpful if space is tight. If you’re into the technical legislation side, you’ll be pleased to hear that Cornerstone Insulating Render also meets the requirements of UK Building Regulations Part L1B. It can be used both internally and externally.
Where are we using it?
We are using the Insulating Render on external walls in the lounge, foyer, and bedrooms of the Cottage. It is a highly insulating product, with a K Value of 0.12, that offers a method of improving thermal performance of a wall whilst also maintaining breathability. The render is permeable and enables moisture to be absorbed and evaporated from the wall, decreasing risk of damp and therefore damage to the fabric of the building.
CLM28 and CLM66 used to plaster half-height in the Cottage lounge.
“A 50mm application will improve the U value of a solid wall by 50% or more, significantly reducing heat loss.”
Whilst the Insulating Render can be left as a textured finished plaster, we are using it for both scratch and float coats in the Cottage, and are finishing it with a fine skim setting coat of Cornish Lime CLM66 fine putty plaster. I absolutely love working with it. It mixes beautifully, and as the aggregate used is extremely lightweight, it’s very easy to work with and in my opinion is much less physically demanding to apply than the Lime Hemp. We have also found that it dries much quicker, perhaps due to the binder being natural hydraulic lime, and can be overcoated after 1-2 weeks depending on the weather and internal temperature.
How much do you need?
1x 9kg bag will cover approximately 0.55m2 at 25mm
Non-Insulating Plasters
As well as the Insulating Render and Lime Hemp plasters outlined below, we have also used non-insulating (standard) plasters in the Cottage on the party wall between the Cottage and the Mistal. The party wall is stone, and earth banked at the base, so needed a breathable material, but as it is effectively an internal wall between the two properties, didn’t need to be insulated. In this instance, we used Cornish Lime’s CLM28 with Fibres (scratch coat), CLM28 (float coat), and CLM66 (setting coat). Standard lime plasters do tend to be cheaper than their insulating counterparts, but can’t be applied in such thick coats, so in some instances what may be saved on initial purchase of the materials can quickly add up in the costs of labour for the application.
Mixing & Preparation
Wetting Down
Before you begin to mix your mortar, it’s a good idea to wet down your wall ahead of time. Very dry or porous walls can suck moisture from fresh plaster, which can cause the plaster to dry out too quickly and crack, delaminate (not stick to the wall) or weaken.
We use a pressure sprayer to wet down our walls - the type you might find at a garden centre. On particularly dry walls, you can wet down the night before you plan to plaster, and again before you being mixing. At the Farm, we tend to wet down before beginning mixing, again just before application, and keep wetting down in sections as we apply the plaster.
Mixing Plasters
Float coat of Lime Hemp in the Bootroom.
How you mix your plaster will depend on the type of lime you’re using, and whether you have bought your plaster premixed or are mixing yourself. Premixed plasters are very simple to mix, and each manufacturer will provide directions on how their plaster should be mixed. We knock up plasters a bag at a time in a flexibucket using an SDS hammer drill with paddle mixer drill bit.
If you’re mixing your own hydraulic lime, you can use a standard cement mixer, or a paddle mixer. It’s important to remember that if you’re mixing your own plaster, that you measure materials by volume rather than weight. Therefore you must use a box or bucket (we use a measuring jug) that you can get a level fill on to measure out. Don’t use something like a shovel as it’s not consistent enough. To mix a 1:3 lime:sand base coat, add one part of sand to the mixer, add one part of lime, and follow with the last two parts of sand. Mix these together for at least 5 minutes before adding your water, and continue mixing for around 20 minutes before using the mortar.
Be sure to add water slowly into a mix, you don’t want to oversaturate the mortar as this can result in the plaster shrinking and cracking as it dries. The saturation of a lime plaster can be deceptive. The lime plasticises as it’s mixed and becomes more ‘fatty’, so it’s always worth mixing for a few extra minutes before deciding to add more moisture. The more water added to a mix, the weaker the final result will be.
Tools
Sponge
‘Optional’ Prep
The following steps are just rough guideline as to how we apply our plaster. In reality, the application process will vary slightly from property to property depending on the type of lime you’re using, and the stage of application you are working at. These notes are just a suggestion, not a hard-and-fast rule, on how to apply plaster in a three-coat system based on our experiences. Please consult the manufacturers guidance on the lime you have purchased, as each plaster is unique and needs to be treated and handled as such.
Remember that lime is extremely alkaline and therefore is caustic. It’s best practise to wear full PPE when mixing lime (ventilated area, mask, goggles, gloves) and we always wear gloves when working with lime. If you’re working with putty or loose plaster mixes, it’s also worth wearing goggles to protect your eyes.
Dubbing Out
Dubbing out is a preparatory process in which hollows or pits in masonry are filled with a very coarse plaster ahead of applying the first base coat. We have successfully used Ty-Mawr Lime Hemp in coarse grade for this process, which has enabled us to attain a much more level final surface than if this process was skipped. Dubbing out can also be used as a method of achieving greater airtightness or draught-exclusion behind bat-and-board insulating approaches.
Harling Coat
A Harling Coat, also known as roughcast, bonding coat or harl, may not always be necessary but can help to stabilise loose surfaces ahead of applying a base coat. Mortar used for harling should be wetter than standard plaster, and is applied very unceremoniously by literally throwing the mortar onto the wall using a flat trowel. The motion is a little like serving a tennis ball, and from our experience with this I don’t think there’s any way of explaining it other than that! If you don’t want to spend out on another trowel, you can do what we do and use an old dustpan! For harl coats, we use Mike Wye’s SecilTek Reabilita Cal
Plastering
Once you’ve knocked up and wet down your wall, the stages of applying the mortar are relatively straight forward. A good tip before you begin plastering is to start at the opposite corner to your strongest hand. For example, because I am right handed, I always begin my plastering in the top lefthand corner, and work across and down the wall. As Will is left handed, he starts at the top right. Working this way means you aren’t crossing over yourself so much, and reduces the risk of you knocking the plaster you’ve already applied.
Usually, you will need to wait at least a week between coats of plaster to allow the previous coat to dry sufficiently before being overcoated. Sometimes it may take longer, and in very cold/wet conditions can take several weeks. If you apply the next coat of plaster too quickly and don’t allow the last coat to dry enough, you will run into problems with uneven shrinkage and cracking. It’s best to keep checking the plaster, and if you’re not sure it’s dry enough for another coat, leave it a few more days. A trick I use is to press your fingernail into the plaster, and then your knuckle. The plaster is ready for it’s next coat when you can press in your fingernail, but pressing in your knuckle doesn’t leave a dent.
Scratch Coat: usually 1:3 lime:sand, haired
The first coat of plaster is called a ‘scratch coat’ because the surface is scratched with a scarifier to provide a key for the next coat. Your scratch coat will be the first base coat of plaster applied to the wall, and can either be applied directly to masonry, or over the top of dubbing out or harl coats. The key to a good base is not to overwork your scratch coat! Lay it on, and leave it alone. The function of this coat is just to have the masonry covered and ready to apply your levelling coats. It won’t be visible once the subsequent coats are applied, so don’t get too caught up in trying to make it perfect.
Hair (traditionally horse hair but now more commonly synthetic) is often added to a scratch coat mix to improve its tensile strength. This improves the longevity of the lime plaster, as it allows the plaster more flexibility as a building moves over time. Hair is particularly important if plastering onto lath, as plaster doesn’t adhere well to timber and the hair helps to build a ‘mesh’ between the laths.
If you’re using a standard lime plaster (i.e. 1:3 mix), the coat should be no thicker than 15mm. Any deeper hollows should be dubbed out prior to scratch coating.
Before you start, respray your wall. If your wall is too dry, it will suck the moisture from the mortar and it won’t adhere properly.
Use your bucket trowel to transfer some mortar from the flexibucket to a plasterers hawk.
Use your plasterers trowel to ‘scoop’ the plaster from your hawk and drop it back onto the hawk. Repeating this action a few times will help to build the structure of the lime in the plaster, and is a process called plasticising. Your mortar is ready to use when it holds to an upturned hawk.
Starting at the top of the wall, scoop the plasticised plaster from the hawk onto your trowel.
Hold the hawk against the wall just underneath where you are applying the plaster, to catch any material you drop from the trowel.
Press the plaster onto the wall in an upward sweeping motion.
Continue to apply the plaster until a large area is covered. Don’t worry about getting it smooth at this stage, just focus on getting the material onto the wall.
Once a large are is covered, use the trowel to smooth off the plaster.
Use a churn brush to beat the surface of the plaster, and then use a scarifier to make a cross-hatched pattern in the surface. This is the key for your next coat, and it’s really important to make sure the whole wall is scratched. This is best done whilst the plaster is still wet.
Depending on how uneven your wall is to start with, you might need to apply more than one scratch coat before moving on to float coats. On some of our walls, we have dubbed out and applied two scratch coats before floating, to give us as smooth a finish as possible.
Float Coat: usually 1:3 lime:sand
Your float coat is the second coat of lime plaster you will apply (unless you are applying more than one scratch coat). The float coat is your levelling coat, and you can use this as an opportunity to straighten walls you weren’t able to fully straighten on the scratch coat. At this point in the process you can introduce a plasterers darby to help you work to a straight edge. Like the scratch coat, if you’re using a standard lime plaster you should work to a maximum depth of 15mm.
Follow steps 1-8 as on a scratch coat
Run your plasterers darby over the fresh plaster, a section at a time to straighten your plaster and show you any areas you may need to apply more material.
Allow the plaster to ‘go off’ (dry out) slightly, before using a plasterers float to rub up the surface, and press in the plaster and open up the surface, adding a light key ready for the final coat. This also helps to counteract shrinkage.
Use a devil float (a wooden float with nails driven through the corners to project about 2mm) to rub over the surface to form a key for the finish coat.
Setting Coat: usually 1:1 lime:sand
Your setting coat, also called a skim, is the finishing coat and the layer of plaster you will see and paint. Because of this, it’s worth taking your time to get this coat right, so I’d suggest starting skimming early in the day to give yourself plenty of time. A setting coat is applied in two very thin coats in quick succession.
The skim coat can be between 2mm and 5mm deep.
Follow steps 1-8 as on a scratch/float coat
Once you have plastered the entire wall, allow the plaster to ‘go off’ (dry out) slightly.
Mix up your second coat of skim, which should be a runnier consistency (natural yogurt is a good reference!)
Repeat steps 2-8, remembering that this is your final coat, so take care!
Allow the second coat to go off, and then use a clean trowel to trowel up. On a cold day, you may need to leave the skim overnight before it’s ready to trowel up.
Finish the coat by working any tooling marks out with a damp sponge, or use a finishing trowel to achieve a polished finish.
Scratch Coat of Insulating Render in the Cottage.
Curing
Lime plaster needs to fully cure before it can be painted. Every lime will have a different curing time, and the manufacturer you have bought your lime from will be best able to advise you on curing time of your lime. As a general rule, you should allow at least 1 month for curing for every 5mm of depth. That means that a wall plastered to 30mm should ideally be left for around 6 months before painting. It’s a long time isn’t it! That isn’t a hard and fast rule though, and as I say it’s worth consulting your manufacturer for curing times.
Paints
Once you’ve gone to all the trouble of plastering your walls in lime, it’s really important to make sure any paints and finishes you use must be highly vapour permeable. Normal paints and emulsions shouldn’t be used with lime plaster, as they contain vinyls which create a seal over the plaster and compromise its vapour permeability and breathability. There are several ranges of paint that are suitable, such as Beeck Mineral Paints, lime washes such as Bauwerk and Graphenstone, and clay paints such as Earthborn, Edward Bulmer and Fenwick & Tilbrook. It is always advisable to ask the brand of paint you’re looking at whether the paint includes vinyls, as even some paints that are marketed as ‘Eco Paint’, such as Earthborn’s Eco Pro Emulsion, actually isn’t as vapour permeable as other options.
We’ll look at paints in more detail at a later date, as we begin to move towards painting in the Cottage, but for now I think this post probably has far more information in it than is easy to absorb! As always, I hope you have found this useful, and it has clarified some of the nuances of lime plastering for you. If there is anything you think I have missed, or if you have any questions, please let me know in the comments below, and don’t forget to join me on Instagram live every Thursday at 12:30pm BST for our Lime Lives series.
Further Reading
Ty-Mawr have some great short clips of each process on their Youtube channel.
Cornish Lime have lots of great information and articles on their website.
