Limewash
Limewash is a matt water based paint suitable for both interior and exterior use.
Limewash has some degree of antibacterial and antimould properties, but is not a totally mouldproof paint. It also discourages some wood boring insects.
Limewash is also one of the planet's cheapest paints.
Making
Limewash is made from lime putty, which can either be bought or made from bagged lime. Readymade putty can give a finer texture than bagged lime, and can be better for interior use.
From Bagged Lime
Take bagged lime from any builders merchant, B&Q etc, (this is hydrated non-hydraulic lime) and mix with water to form a putty. Leave the putty in a sealed container or under water for a couple of days if you want to optimise the texture. Dilute it 50/50 with water. You now have limewash.
From Putty
Dilute the putty 50/50 with water. You now have limewash.
Coats
I've nearly always had good results with 2 coats. If you get the mix too thin it would take more, but this is easy to correct on the 2nd coat.
On horizontal exterior surfaces it is possible to use just 1 coat. A fairly thick layer of the thin paint is spread on and not brushed out. (Don't thicken the paint, that causes cracking.) This has been enough to make bare grey concrete a good full bodied white.
Surfaces
It is sometimes said limewash won't stick as well to concrete, gypsum, and some other surfaces. However I've not had a problem with this, so would treat such reports with a good pinch of salt.
If a surface is encountered that it doesn't stick to, the addition of 0.5% to 0.75% linseed oil to the paint makes it stick to pretty much anything.
Durability
It doesn't rub off onto clothes in normal use. Brushing against it daily is not a problem. Only if you scrape it hard would a little dust come off, so its no good for schools & prisons.
Limewash is physically weak when first applied, it takes a day or 2 to harden up. In very cold weather this could take longer.
Application
When first applied the paint looks excessively thin. Don't worry about this, it bodies up when it dries, then it bodies up again when it cures. Don't be tempted to thicken the paint, this would cause cracking.
Stir it up frequently during painting, as it tends to settle. It is thus recommended to paint from a small container so that stirring only takes a moment. I stir about every 3rd brushful, otherwise it settles quickly.
Leave the first coat a day before applying the second. If you apply the 2nd as soon as the first is dry, the first coat would come off. This is because curing takes longer than drying, the paint is not cured as soon as its dry.
Uncured paint is mostly just damp powder. This gradually chemically reacts with CO2 in the air to form a hard layer of paint.
Don't paint outdoors when rain is imminent, as it takes a day or so to cure enough to firm up. Slight drizzle seems harmless, but heavy rain can wash fresh paint off.
Very absorbent surfaces should be dampened before application.
The liquid paint is thin and ungelled, so is prone to dripping. However limewash has one great property: any freshly dried drops can simply be hosed away. Thus a dustsheet is only wanted where hosing would not be practical. Drops left till next day won't wash off.
Cured paint can be removed with acid if the underlying substance doesn't object.
Exterior painting should not be done when frost is expected. The paint needs to cure before handling frost.
Repainting
When limewash reaches end of life, it can become a bit more powdery. A new coat of limewash is all that's needed, this rebonds the old paint layer as well as applying a new paint layer.
Repainting with limewash
Brush the paint to remove anything loose, dampen if necessary, and repaint.
Repainting with emulsion
Applying a thin coat of very dilute PVA (eg 5:1) will maximise the life of the emulsion coat by bonding the lime coat.
When PVAing before painting, use the minimum necessary amount of PVA, you don't want to form a slick surface.
Colour
Limewash is nearly always used white. It can be coloured if desired, but colouring comes with a few caveats:
- Limewash produces light to medium colours. Dark and saturated colours are not possible
- Alkali stable pigments should be used. Unstable pigments would fade.
- Limewash lightens during drying & curing
- Mixing a second matching batch is close to impossible, so be certain to make enough coloured paint in one go.
- Weighing the pigment exactly does not ensure identical colour, as grain size affects colour saturation, and grain size is typically not controlled between batches.
Traditional Pigments
Earth gives colours ranging from broken white to brown. Subsoil is most stable. Soil pigments have been used for thousands of years, and are still standard in many parts of the world.
Iron oxide gives pink or an unsaturated red.
Laundry blue gives a light blue, but this is not necessarily alkali stable. The colour may fade in time. This has been used as a colouring agent for interior paints for over a century.
Issues
Lime is alkaline, and mildly irritant to skin. Take care not to splash lime into eyes. If this occurs it must be washed out right away, never be tempted to leave it just because it doesn't hurt.
Bagged builder's lime gives a surface finish that is not as fine as interior emulsion.
Troubleshooting
If the paint cracks, it was mixed too thick. It is not necessary to remove the cracked coat, just paint some thinner limeweash on and it'll fill in the cracks and bond it together properly.
If it takes multiple coats to become full bodied, it went on too thin.
Unusual Problems
If your paint turns out non-white, you have a batch of lower purity lime. The impurities are tinting the paint. Use the type of lime recommended here for white paint.
If the paint sets in the tin during painting - a very rare problem - you either bought hydraulic lime by mistake, or mixed in a pigment that is pozzolanic, such as brick dust.
Smoothing Paint
An uncommon method of lime painting can both smooth and paint in one coat. This is usable on concrete surfaces such as exterior walls, where it is desired to improve the rough surface texture.
Instead of diluting the putty, apply it as is using a semi-flexible plastic or rubber edge. Just wipe it on so it fills the indentations.
The result is that the surface roughness is filled to a fair extent, but not completely. Smoothness can not be achieved with one coat because the lime in the indentations shrinks and microcracks as it dries. The result is smoother, but not smooth. When the surface is repainted in several years time, these microcracks are filled and more material is deposited in the hollows, so the surface becomes smoother at each repaint.
A rubber edged grout spreader can be used, but pick one with a stiffer rather than a soft edge, and expect tool wear.
This method uses more lime than standard painting, but its extremely cheap. A £6 bag goes a very long way.
Only one coat is needed to get a full bodied white coat with this method. If you want to apply a 2nd coat for a smoother result, leave the first coat several days to harden first.