Top Tips from Cait: Hydrophobix & Hydrocalce
Neither Hydrocalce nor Hydrophobix are coatings. They are instead materials that have a chemical reaction with the dry silicate – in this case the lime – that allow the material to continue to breath as they are designed to do. Therefore, the first thing to do is to make sure that your material is utterly dry. We recommend 3 days’ drying time, however if the rest of the building is being plastered or painted, then there is going to be high humidity in the building; similarly if the weather has been very cold and/or very wet you should then allow a longer period. While we are thinking about the cold, although these products aren’t affected by cold in the same way as paint, they should not be applied at a temperature below the dewpoint. When the temperature drops below the dew point, the air can no longer hold all its moisture as vapor, and condensation occurs leaving moisture on the surface and this can mark the plaster or slow the drying and therefore the reaction.
When applying these waterproofing products, the aim is to saturate the surface, ensuring that the plaster won’t take up any more of the material. Working from bottom to top I personally like to use a good-sized brush, applying particularly the first coat very liberally, and then using a sponge to mop up and move around any excess; others are happy to use a roller or a sprayer, this is just my own technique.
The first coat will take most material, then as you apply the second and third coat the absorption will lessen but you need to still apply enough material that the plaster is saturated. If the walls are small, I might apply to more than one wall at a time but generally I work wall by wall, applying the three coats in succession before moving on to the next. Neither Hydrocalce nor Hydrophobix will compensate for poor application and attention to smoothing and polishing the plaster with the aim of closing the pores of the plaster is important. However, even the best of applicators will have some small pores that may remain open. This is important to remember; as the waterproofing material is drying there may be some of these tiny pores in that saturated surface that hold moisture, and yet at the last minute spit out a tiny dribble of the product as evaporation continues to happen. These tiny dribbles can leave marks on the plaster if left, so it is important that you don’t walk away and leave the drying wall till you are sure the material is drying in completely.
Pictures courtesy of Carl Ferri from C Ferri Interiors. London (@ferridecs on Instagram)