How to Waterproof Plaster
Plaster can be found both inside and outside homes. Exterior cement plaster is used in the construction of buildings, and it doesn’t need to be waterproofed because it is made of cement. It is also a very strong material, and even without paint, it can withstand extreme outside conditions.
Plaster used on the inside of homes is a different material altogether. It is water soluble and, without care, can succumb to moisture and humidity. Although interior plaster can never totally be waterproofed, there are some steps that you can take to create a stronger moisture barrier.
If you plan on painting your interior plaster, a traditional primer and paint treatment will create a sufficient moisture barrier against the elements. Remember, you cannot treat the plaster in any way to create a water barrier from the inside of the wall. If you have a leak, even a small one, you should repair it promptly to reduce the risk of water damage to your plaster walls.
Step 1 - Coat With Primer
Before painting the wall, apply one coat of a heavy-duty primer. This will not only create a beautiful finished look to your paint job, but it will provide the foundation layer of your paint barrier.
Step 2 - Apply the Paint
A standard interior paint will work fine over your primer; however, if you are looking for stronger water protection, use a heavy-duty exterior paint instead. Exterior paints have added ingredients that help them withstand the elements. This will fortify your walls even further.
If you don’t want to paint the plaster but still want to achieve a moisture barrier, you can apply a light coat of linseed oil to the walls. Again, this treatment will not eliminate the effects of water damage if water meets the wall from the inside.
Step 1 - Lightly Sand
Lightly sand the plaster surface to a beautifully smooth finish before adding the oil. This will allow it to absorb more efficiently.
Step 2 - Apply the Linseed Oil
When deciding on a paintbrush to buy, always try to find a high-quality variety. The natural bristles of a better brush will help the oil application remain even throughout.
Starting from the top corner of the wall, brush a very light coat of linseed oil over the entire surface. Make sure that the application of oil is light, as you don’t want to end up with a greasy wall. The naturally porous nature of the plaster will cause the oil to sink in. After a few hours you shouldn’t be able to tell it was treated at all.
WARNING: Linseed oil is highly flammable, and paintbrushes or rags soaked in it will suddenly set on fire if put in a warm area. Carefully place any tools you use to apply it in water-filled containers once you're finished with them.
The process is as simple as that! In just a few steps you have treated your plaster walls with the most effective waterproofing procedures available.