Before painting, first tell the cracks apart: fine hairline and craze cracks are cosmetic and you fill, bond and over-coat them; straight or stepped cracks wider than about 0.5 mm are structural and must be repaired by a specialist first — paint will not hold a moving crack together. Fill and paint a cosmetic crack and it stays hidden; paint over a structural crack and it reopens through the film. This guide shows how to identify, prepare and coat cracks so the repair lasts, using the right Rhinoluxe primer.
What kind of crack do you have?
Hairline and network “craze” cracks are shallow, sit in the plaster or paint film, and stay stable; true cracks wider than about 0.5 mm that run straight, or step diagonally through the wall, are structural and move. The South African standard defines a hairline crack as 0.5 mm or less. Craze cracks form a fine hexagonal net and do not open and close. Structural cracks come from foundation movement, thermal movement or masonry shrinkage, and they will keep working — which is why the two are treated completely differently.
What causes cracks in plaster and paint?
Cosmetic cracks usually trace back to how the plaster or paint was applied — a too-rich plaster mix, plaster that dried too fast, layers applied too thick, or paint over-applied in runs and puddles — while structural cracks trace back to the building moving. Crazing typically comes from over-trowelling a high-cement mix or sand with too much dust; drying-shrinkage cracks come from plaster losing water too quickly in sun or wind. Paint-film cracking comes from thick, brittle or over-thinned coats that cannot flex. Structural cracking, by contrast, comes from foundations settling, thermal expansion of the roof and walls, or masonry shrinkage — forces a coating cannot resist. Knowing which family you are dealing with points you straight to the right fix: surface treatment for the first, structural repair for the second.
How do you fix hairline and craze cracks?
Most fine cracks are handled by a flexible, higher-build coating system — clean out the crack, bind the surface, and over-coat with a paint that has enough film build and flexibility to bridge and hide it. Rake loose material out of the crack and dust it off. Bind a chalky or powdery surface with Clear Bonding Liquid so the repair grips. Where fine cracking is widespread, a medium-build texture or sheen coating hides it far better than a thin flat paint; on persistent hairlines a reinforcing membrane can be embedded during coating for extra insurance.
How do you fill and prime a crack?
Open the crack slightly, fill it with a suitable crack filler, sand flush, then prime the repair so the filler does not flash through the topcoat as a dull patch. A filled but unprimed repair absorbs paint differently to the surrounding wall and shows as a mark. Seal and even out the porosity of the patch with a primer/undercoat — MultiPrime or Universal Undercoat — before the topcoat, so the finished wall is uniform in colour and sheen.
How do you know if a crack is structural?
Structural cracks are generally wider than 0.5 mm, run in straight vertical or horizontal lines or step diagonally, and often continue through both plaster and the masonry behind. They may widen over time, appear at wall junctions, or open and close with temperature. These originate in the wall, not the plaster, so repairing the plaster alone does nothing — the crack simply returns. If a crack is growing, wide, or you are unsure, treat it as structural until a professional confirms otherwise.
What do you do about structural cracks?
Structural cracks need a specialist to establish and fix the cause first — painting over them is pointless because they keep moving and reopening through any coating. Remedies can include structural repair, or converting the crack into a designed movement joint that is then hidden with a cover strip or sealed with a flexible sealant. Only once the movement is dealt with does the surface get re-plastered, primed and painted. Skipping this step means the crack telegraphs straight back through your new paintwork.
How do you paint the repaired area?
Once cracks are filled or repaired, the patch primed, and the surface clean and dry, coat the wall edge to edge in a quality exterior or interior paint matched to the surface. Priming the repairs first is what prevents patchy “touch-up” differences in the finished wall. Work corner to corner rather than spot-patching so the sheen and colour stay uniform, and paint in the correct conditions — not in extreme heat, cold or damp. See the surface preparation guide for the full prep sequence and the paint problems guide for other defects.
When to get help
If cracks are wide, stepped, growing, or reopening after previous repairs, get a professional assessment before painting — that is a structural signal, not a paint job. Money spent repainting over active movement is wasted. For advice on the right filler, bonding and coating for stable cosmetic cracks, or which primer to use on a repaired wall, call +27 84 985 6141 or visit our paint shops in Table View and Edenvale.
