Ty-Mawr Premixed Lime Base Coat plaster is premixed and ready-to-use, it simply requires 'knocking-up' prior to use.
It is made from our high calcium/non-hydraulic lime blended with carefully selected, high quality aggregates which help to ensure good workability as well as minimal shrinkage.
It is suitable for internal plastering but can also be used for external rendering on 'soft' substrates given appropriate application and protection.
It requires exposure to Carbon Dioxide in the presence of moisture to harden. It is therefore important to use these plasters at an appropriate time of the year and to protect them from drying too rapidly.
Application guides are printed on the bags and available below, we also have an excellent telephone support service for customers as well as courses at Ty-Mawr or on your site, so don't be put off from using them - the results are well worth it!
- Properties : the most flexible and vapour permeable/breathable of the lime product family, so highly suitable for softer sybstrates and for solid wall or breathing construction buildings.
- Used for : external plastering onto soft substrates such as strawbale, soft brick/stone, cob etc and almost always internally refer to "Choosing a Lime" in Documentation below.
For assistance with calculating the materials you require, please go to our quantity calculators where you will be asked a series of questions to help you to assess your needs or call one of our experienced sales team.
- Availability : This product is available with or without hair. It is available in 1 tonne bulk bags, 25kg bags or 20 litre buckets.
This product qualifies for our bulk order discounts, the more you buy, the greater the discount!
- Technical Documents
- Application Advice
Click here for our video-based application guide
gives a consistent, accurate mix thus removing the potential for any mixing and gauging errors.
minimises waste, enables you to 'knock-up' as much as you require in a day. Anything left over can be reused, so nothing going to waste.
high quality aggregate selection produces a very workable 'fatty' mix, minimising shrinkage.
helps to keep the site organised and clean.
their ability to control moisture makes them highly appropriate for use with other ecological materials.
it is generally accepted that lime plasters can improve the internal environment, regulating humidity by absorbing moisture, unlike modern plasters.
- 1 tonne covers 40m2 at 6-9mm thick
- 25Kg Bag\20L Bucket covers 1m2 at 6-9mm thick
What is the best way to apply lime plaster?
This advice applies to both hydraulic and putty limes. Although we recommend that putty limes are generally best for internal work. It is extremely important that application guides for the different products should be followed but in general internal lime plaster should be applied as 3 coats:
1st coat: Scratch Coat. Use well graded sands and plenty of hair. Coat should be 6 – 9mm and diamond scratched.
2nd coat: Float Coat. Use well graded sands and plenty of hair. Coat should be 6 – 9mm and floated off.
3rd coat: Finish Coat. Use fine sand and apply 2 tight coats of around 1.5mm.
For other applications and different substrates, please see the application guides in the Documentation section. Substrate preparation, mixing, all procedures - scratching, floating etc and tending must be done correctly and at the right times!
How many days should I leave between scratch, float and finish coats?
Ideally 2 weeks should be left between scratch and float coats, and 1 week between float and finish. The minimum is 5 days. But, every site and every application will be different, expereince is crucial - it needs to be monitored daily as it is ready for its next coat when you can push a fingernail in but not a knuckle - call for advice if you are unsure!
Can I use lime plaster on ordinary gypsum plasterboard?
No, conventional plasterboard does not have a coarse enough surface to provide a good key for lime plaster. Also it is normally necessary to wet the substrate to minimise suction prior to lime plastering. Plasterboard does not react well to be heavily wetted. We suggest using 15mm Ty Mawr Wood Wool board as a ‘breathable plasterboard’
How do I fix wood wool boards?
Wood wool boards can easily be cut to shape using an ordinary site saw. The number of fixings and spacing of battens to support the boards will depend on the thickness of board to be used. Information on this can be supplied on request. Boards can be fixed using ordinary sheradised or nickel plated screws the length of which should be the thickness of the board plus a minimum of 25mm. For external application or where screwing into green oak, it may be advisable to use stainless steel screws. A fixing washer should be used to avoid the screw pulling through the board.
How do I plaster onto wood wool board?
Please refer to the application guide for the specific plaster that you are using or call for advice.
Can I attach glass fibre mesh to boards or do I have to embed it in the plaster, and can it be cut down as a scrim?
Glass fibre mesh should be pushed into a tight first coat and then the rest of the first coat applied over it. This prevents movement of the mesh. We would recommend that mesh is used across the whole area as oppose to simply scrimming the joints.
How do I skim onto old plaster?
Remove old limewash and sand off any wall papering residues. If a conventional paint has been used this must be removed. A devil float should be used to key the old plaster and slurry coat made from finish plaster and water and applied with a brush. You should then apply your skim with finish plaster applied in 2 tight coats of 1.5mm thick.
How do I use and apply roughcast?
It is normal to apply two coats of plaster when doing a rough cast. The first scratch coat should be well haired and float applied to a depth of 7-8mm. A devil float can be used to rough up the surface to give a good key for the roughcast. The roughcast can be applied either with a traditional harling trowel or with a spray gun. Please contact us for information on hiring spray guns.
Can I apply lime plaster to metal lath?
Yes but make sure it is stainless steel and is fixed with stainless steel fixings. Both Expanded Metal Lathe and Rib Lathe are both suitable. We would only recommend their use when there is no other alternative. Make sure the base coat is very well haired.
Replacing old plasters
Do I need to remove all old plaster?
If the old plasters are themselves lime based plasters, does all the plaster have to be removed?
It is often the case that unnecessary damage, waste and energy is used removing perfectly good old lime plasters. It is perfectly possible to patch in to these old plasters and thereby retain some of the authentic original fabric of the building.
Building Regulations require you to submit an application if renovating more than 25% of the surface area of a thermal element – “Renovation of thermal elements means either the provision of a new layer in the thermal element or the replacement of an existing layer of the thermal element. When renovation to more than 25% of the surface area of the thermal element is proposed it will be necessary to submit a Building Regulations application and for upgrading work to be carried out so that the element achieves U values provided within the Approved document”. (Building Control Part L 2006). This often means that that building control may require the building to be dry lined with the inclusion of insulation.
Can I lime render or lime plaster over concrete blocks?
Yes but there are some golden rules that should be adhered to:
Rendering onto concrete block externally:
Suction must be controlled either by damping down or by the use of ‘Primal’. This is particularly important when plastering onto insulation or lightweight blocks
Damp down or treat substrate with one coat ‘Primal’ thinned 10:1 with water. Allow to dry and just prior to plastering paint on second coat ‘Primal’ thinned 20:1 with water.
Apply first coat of hydraulic lime to 6-9mm. If high suction block/insulation block apply first coat while ‘Primal’ still tacky. If ordinary block, damp down background before starting. Sand : lime ratio 2 :1. Sand to be standard Ty-Mawr scratch sands with hair added to give tensile strength.
Hydraulic Lime top coat to be applied 6-9mm with sand : lime ratio 2.5 : 1 but using a finer sand to that above. Finish as required.
Plastering onto concrete block internally:
Surface treatment as above
Use the three coat plastering system described above.
Why do I need to protect lime work?
Lime products need to carbonate (not just set) which they need to do by absorbing Carbon Dioxide in the presence of moisture. It is therefore important to create the correct conditions for carbonation which comes through experience but is vital to the success - refer to the application guides for each product for details on protection - it is one of the most important points to ensure a successful result!
Different limes, different applications, different backgrounds (even in the same building), different times of the year and different elevations on the same site will all require careful attention and possibly different measures.
Most of the failures that we see are down to a lack of appropriate tending to aid carbonation as even a poorly specified/selected lime type can actually succeed if applied and cared for appropriately after all we did not historically have the range that we have now! As a result, we concentrate on this particular aspect on our training courses and in our application guides and are happy to discuss any aspect of it with our customers - call 01874 611350! We want your job to succeed and most do!