We always see a huge number of queries on the subject of mist coating and the use of PVA so I have decided to start a thread on the subject. I have not covered problems such as surface contamination, over-polished plaster, walls that have damp, problem walls, etc., but hopefully this does cover the very basics of preparing bare or newly plastered, boarded and rendered walls for decorating and will prove to be helpful. If so, perhaps it will be made into a 'Sticky' by Admin. Otherwise, if members wish to reply, I will try to incorporate any additional useful information into this post so that it stays towards the top of the thread.
PVA
There have been a number of debates on this forum, and also on many worksites, between Decorators, Plasterers, Builders, etc., regarding the use of PVA as a primer/sealer for bare plaster and masonry. There are probably as many people who recommend it's use as there are that condemn it.
Whether you are pro or anti PVA, there is little doubt between the Professional Decorators here that it causes a whole host of problems when used in conjunction with paints, and should therefore be avoided completely.
It sits on the surface of the plaster, rather than being soaked into it like a mist coat does, and dries to leave a hard, shiny film. This shiny surface subsequently causes the paint to spread over it far too thinly, ultimately stopping it adhering properly. Whilst the paint will often dry on top of the PVA successfully, further down the line, most people find that they end up with flaking, cracking and peeling paint. On other occasions, when it is time to redecorate, the existing 'good' paint will lift away from the PVA when new paint is applied on top of it.
Some PVA manufacturers even state on their products/datasheets that it is not suitable for use with water-based paints. (Picture can be seen on my Profile page).
Even with a whole host of Specialist Primers on the market, there is still no product that says it is suitable for use on PVA painted surfaces. Some oil-based products will adhere to it but, as far as I am aware, no manufacturer will guarantee this.
Whilst PVA is a superb product, with numerous uses when used as directed, the above reasons should be enough to convince you that it should be avoided when it comes to decorating.
THE MIST COAT
There are a variety of sealers for new plaster available to purchase but the mist coat is the cheapest, and most commonly used method, for sealing new or bare plaster that you wish to paint.
The basic principle is to apply a first coat of emulsion paint, diluted with clean, cold water, to any new or bare plaster in order to seal it. The rate of dilution will vary from product to product, due to their differing viscosities. It can be anything between 10% water, which is generally what paint manufacturers recommend, up to 50% water. This higher dilution ratio is rarely required and would only be for very thick Trade Emulsions, which are not usually used for mist coating because of their higher cost.
The best type of product to use is a Non Vinyl Contract Matt Emulsion.
This type of emulsion contains no vinyl and so can be applied to bare plaster that is still drying out. The fact that it contains no vinyl allows any moisture still in the plaster to escape through the paint film, meaning you can begin the mist coating a little sooner than with other emulsions.
The most common dilution ratio for this type of paint, again depending on specific brand thickness, is between 15%-30% water.
It is always wiser to allow new plaster to dry completely before applying any paint, but this type can be used on areas where the plaster is still drying, thus helping speed up the decorating process.
It is very important is to ensure that, before you apply your finishing coats of paint, the plaster has completely dried out through the Contract Emulsion. This can usually be confirmed by the shading of the surface - damp areas will appear a little darker than dried out areas.
Any type of emulsion can be used on top of Contract Emulsion or further, full thickness coats of the product can be used as your finishing coats. It is wiser to only use this paint as a finish coat for ceilings as it is not hard wearing, is easily marked and cannot be wiped with a wet cloth.
Many people believe that Vinyl Matt or Vinyl Silk emulsions cannot be used to mist coat. This is not the case, and both can be used successfully providing some stricter guidelines are followed.
When using a Vinyl emulsion as a mist coat, you must ensure that the plaster is completely dry before starting as any moisture will be trapped behind the paint film, causing it to peel as it tries to escape.
Vinyl Matt can be used, diluted as mentioned above, and then painted over with matt, soft sheen (satin), silk or acrylic eggshell as your finish coats.
Vinyl Silk is the last choice of emulsion to mist coat with, and should only really be used if your intended finish coats are also to be silk. The reason being, the shiny surface of dried silk emulsion can cause cracking and crazing if your top coat is a different type of paint.
Deciding on the correct dilution for the product you wish to use may take a little experimentation. Start by diluting it to the lower ratio, apply a small amount to an inconspicuous area and allow to dry fully. Once dry, press a piece of sticky tape firmly onto the paint and then pull away quickly. If the paint stays adhered, it should be fine to proceed. If not, you will need to add a little more water and carry on testing until you find the right mixture that adheres to your surface.
Professional Decorators can usually tell from experience what the correct consistency for a mist coat will be but the above test is very helpful, if a little time consuming, for the DIYer.
MASONRY SURFACES
The same principle applies when painting new or bare masonry surfaces such as render, concrete blocks, roughcast, pebbledash, etc., although the surface must be completely dry before starting with a diluted Masonry Paint being used for the mist coat.
This type of paint does not need to be diluted as much as some interior emulsions and is generally required to be mixed with between 10%-20% water.
If diluting Textured Masonry Paint, ensure it is thoroughly mixed before and during use, as the fine particles can start to settle at the bottom of the container when water is added.
It is also wise to dilute masonry paint with approximately 5% water in order to aid flow and ease application when applying to previously painted textured/roughcast surfaces.
Friable, chalky or weathered surfaces should be sealed with a suitable Stabilising Solution before painting.
DRYLINED WALLS AND COVING
There are specific Drywall Sealers available for bare plasterboard which should be applied following the manufacturer's instructions.
You may also seal the plasterboard with a mist coat of emulsion. Doing this should not require as much dilution as with bare plaster due to the fact that plasterboard is not usually as porous. The most porous areas are where boards have been taped and filled with joint compound, so this is where you should pay most attention to the adhesion of a mist coat.
SIZING FOR WALLPAPER
When sealing new or bare plaster for wallpapering, it is only really necessary to use Size. This can either be a specifically designed Wallpaper Size or a weak mix of wallpaper paste, diluted in accordance with manufacturers instructions.
You could also use a mist coat of emulsion for sealing the plaster instead of size. This will ensure a uniform colour beneath your wallpaper but is not essential as sizing is much quicker, easier and is designed for the task.
If you wish to paint onto a bare plaster wall after removing wallpaper, you should either spend lots of time ensuring all traces of wallpaper adhesive are removed or coat the surface with a product specifically designed for sealing old wallpaper paste.
Cheers, and good luck,
mrH
PVA
There have been a number of debates on this forum, and also on many worksites, between Decorators, Plasterers, Builders, etc., regarding the use of PVA as a primer/sealer for bare plaster and masonry. There are probably as many people who recommend it's use as there are that condemn it.
Whether you are pro or anti PVA, there is little doubt between the Professional Decorators here that it causes a whole host of problems when used in conjunction with paints, and should therefore be avoided completely.
It sits on the surface of the plaster, rather than being soaked into it like a mist coat does, and dries to leave a hard, shiny film. This shiny surface subsequently causes the paint to spread over it far too thinly, ultimately stopping it adhering properly. Whilst the paint will often dry on top of the PVA successfully, further down the line, most people find that they end up with flaking, cracking and peeling paint. On other occasions, when it is time to redecorate, the existing 'good' paint will lift away from the PVA when new paint is applied on top of it.
Some PVA manufacturers even state on their products/datasheets that it is not suitable for use with water-based paints. (Picture can be seen on my Profile page).
Even with a whole host of Specialist Primers on the market, there is still no product that says it is suitable for use on PVA painted surfaces. Some oil-based products will adhere to it but, as far as I am aware, no manufacturer will guarantee this.
Whilst PVA is a superb product, with numerous uses when used as directed, the above reasons should be enough to convince you that it should be avoided when it comes to decorating.
THE MIST COAT
There are a variety of sealers for new plaster available to purchase but the mist coat is the cheapest, and most commonly used method, for sealing new or bare plaster that you wish to paint.
The basic principle is to apply a first coat of emulsion paint, diluted with clean, cold water, to any new or bare plaster in order to seal it. The rate of dilution will vary from product to product, due to their differing viscosities. It can be anything between 10% water, which is generally what paint manufacturers recommend, up to 50% water. This higher dilution ratio is rarely required and would only be for very thick Trade Emulsions, which are not usually used for mist coating because of their higher cost.
The best type of product to use is a Non Vinyl Contract Matt Emulsion.
This type of emulsion contains no vinyl and so can be applied to bare plaster that is still drying out. The fact that it contains no vinyl allows any moisture still in the plaster to escape through the paint film, meaning you can begin the mist coating a little sooner than with other emulsions.
The most common dilution ratio for this type of paint, again depending on specific brand thickness, is between 15%-30% water.
It is always wiser to allow new plaster to dry completely before applying any paint, but this type can be used on areas where the plaster is still drying, thus helping speed up the decorating process.
It is very important is to ensure that, before you apply your finishing coats of paint, the plaster has completely dried out through the Contract Emulsion. This can usually be confirmed by the shading of the surface - damp areas will appear a little darker than dried out areas.
Any type of emulsion can be used on top of Contract Emulsion or further, full thickness coats of the product can be used as your finishing coats. It is wiser to only use this paint as a finish coat for ceilings as it is not hard wearing, is easily marked and cannot be wiped with a wet cloth.
Many people believe that Vinyl Matt or Vinyl Silk emulsions cannot be used to mist coat. This is not the case, and both can be used successfully providing some stricter guidelines are followed.
When using a Vinyl emulsion as a mist coat, you must ensure that the plaster is completely dry before starting as any moisture will be trapped behind the paint film, causing it to peel as it tries to escape.
Vinyl Matt can be used, diluted as mentioned above, and then painted over with matt, soft sheen (satin), silk or acrylic eggshell as your finish coats.
Vinyl Silk is the last choice of emulsion to mist coat with, and should only really be used if your intended finish coats are also to be silk. The reason being, the shiny surface of dried silk emulsion can cause cracking and crazing if your top coat is a different type of paint.
Deciding on the correct dilution for the product you wish to use may take a little experimentation. Start by diluting it to the lower ratio, apply a small amount to an inconspicuous area and allow to dry fully. Once dry, press a piece of sticky tape firmly onto the paint and then pull away quickly. If the paint stays adhered, it should be fine to proceed. If not, you will need to add a little more water and carry on testing until you find the right mixture that adheres to your surface.
Professional Decorators can usually tell from experience what the correct consistency for a mist coat will be but the above test is very helpful, if a little time consuming, for the DIYer.
MASONRY SURFACES
The same principle applies when painting new or bare masonry surfaces such as render, concrete blocks, roughcast, pebbledash, etc., although the surface must be completely dry before starting with a diluted Masonry Paint being used for the mist coat.
This type of paint does not need to be diluted as much as some interior emulsions and is generally required to be mixed with between 10%-20% water.
If diluting Textured Masonry Paint, ensure it is thoroughly mixed before and during use, as the fine particles can start to settle at the bottom of the container when water is added.
It is also wise to dilute masonry paint with approximately 5% water in order to aid flow and ease application when applying to previously painted textured/roughcast surfaces.
Friable, chalky or weathered surfaces should be sealed with a suitable Stabilising Solution before painting.
DRYLINED WALLS AND COVING
There are specific Drywall Sealers available for bare plasterboard which should be applied following the manufacturer's instructions.
You may also seal the plasterboard with a mist coat of emulsion. Doing this should not require as much dilution as with bare plaster due to the fact that plasterboard is not usually as porous. The most porous areas are where boards have been taped and filled with joint compound, so this is where you should pay most attention to the adhesion of a mist coat.
SIZING FOR WALLPAPER
When sealing new or bare plaster for wallpapering, it is only really necessary to use Size. This can either be a specifically designed Wallpaper Size or a weak mix of wallpaper paste, diluted in accordance with manufacturers instructions.
You could also use a mist coat of emulsion for sealing the plaster instead of size. This will ensure a uniform colour beneath your wallpaper but is not essential as sizing is much quicker, easier and is designed for the task.
If you wish to paint onto a bare plaster wall after removing wallpaper, you should either spend lots of time ensuring all traces of wallpaper adhesive are removed or coat the surface with a product specifically designed for sealing old wallpaper paste.
Cheers, and good luck,
mrH
