Mould Resistant Paint
The best solution to mould is to prevent it by drying the room. Mould resistant paint is a good second line strategy. It is possible to buy mould resistant paint, but it is also easy enough to make the paint you already have mould resistant.
These work with both water and oil based paints.
Chemicals
Any one of these gives paint significant mould resistance:
- Aspirin
- Copper compounds
- Alum
- Zinc sulphate
- Borax
Combine more than one additive for maximum mould resistance.
- With alkyd or oil based paints all additives need to be very finely powdered.
- Water based paints dissolve the above additives, making this less important.
- Lumpy soluble additives such as soluble aspirin should be added, time given to dissolve, then the paint mixed thoroughly.
When using aspirin, the salicylate reacts with water to form salicylic acid. 16 aspirin tablets per 2.5 litres of paint is suitable.
Some copper compounds are toxic if ingested in more than miniscule amounts, so wash hands afterwards. Too much copper can cause noticeable paint discolouration.
For outdoor paint, ferrous sulphate can be used to prevent algae.
Water soluble additives can leach out of the paint in time. Nonetheless the author's first use of this method (in a bathroom with aspirin) remained effective over a decade later. Don't wipe down wet walls, let them dry and the additive is not lost. A nonsoluble addditive such as copper powder is likely to last better outdoors.
Care
Most of these additives are water soluble, which raises the potential for them to simply be leached out of the paint by repeated condensation. Having used such formulae for years I can only say it works anyway, perhaps partly because the condensation evaporates away, leaving the mould proofer where it is, or perhaps for other reasons. However, one simple rule applies if you want it to work long term: don't repeatedly wipe the wall down when its damp, if you do you'll remove the mouldproofer.