To prepare an exterior wall for waterproofing, clean off dirt and algae, treat mould, repair cracks over about 2 millimetres (mm), prime bare or chalky plaster, and confirm the wall is dry before coating. Skipped prep is the single biggest cause of waterproofing failure, so this stage decides whether the coating reaches its full 10-year life or peels within a season.
How to prep a wall for waterproofing (step by step)
- Clean the surface. Wash off dirt, dust, chalk and algae with water and a stiff brush. A waterproof coating only bonds to a clean, sound surface.
- Treat mould and algae. Kill mould and algae with a suitable biocide or steriliser and let it work before rinsing. Painting over live growth lifts the coating within months.
- Repair cracks. Fill cracks over about 2 mm with a flexible filler. Waterproof membranes bridge hairline cracks but not structural gaps.
- Prime. Prime bare, porous or chalky plaster with Solvent Plaster Primer to control absorption and lock the surface down.
- Check it is dry. Confirm the wall is dry and free of rising damp before coating. Trapped moisture pushes the film off from behind.
What needs to be done before waterproofing?
Before any waterproofing coat goes on, the wall must be clean, dry, crack-free and primed. On a new plaster wall, allow the plaster to cure fully first, then prime. On a previously painted wall, remove all loose and flaking paint. On a wall with active damp, find and fix the moisture source — a waterproof topcoat over a wet wall traps the moisture and blisters.
Which products are best for wall waterproofing prep?
Three products cover most wall prep: a biocide or steriliser for mould, a flexible filler for cracks, and the correct primer for the surface. Use Solvent Plaster Primer on new cement and plaster. Then apply the waterproofing coat — Shield on exterior walls, or Damp Proof on an interior damp wall.
Preparing bathroom and interior walls
Interior wet-area walls need the same clean-dry-prime sequence, plus attention to the moisture source. A bathroom or kitchen wall takes a washable UltraLux or a water-based High Sheen Enamel; wet areas behind shower tiles are tanked with Megaflex cementitious waterproofing, not paint alone.
Common prep mistakes
The four mistakes that cause callbacks are: coating over algae, skipping the primer on chalky plaster, filling cracks with a rigid filler, and coating a wall that is still damp. Each breaks the bond between the wall and the film.
Frequently asked questions
Do you need to prime a wall before waterproofing? Yes on bare, porous or chalky plaster. Priming controls absorption and locks the surface so the coating bonds.
Can you waterproof a damp wall? No. Fix the moisture source and let the wall dry first, or the coating traps the damp and blisters.
Where to buy
Shop Preparation products and the Waterproofing range, or read the complete waterproofing guide. Trade pricing: +27 84 985 6141.
