Painting, Varnishing and Surface Protection
What this domain tests
Painting questions cover paint constituents, surface preparation, the correct coat sequence, coverage rates, varnish and polish types, and the diagnosis of paint failures. Protecting steel and damp-prone masonry is a recurring theme.
Core rules and concepts
- Paint has four parts: base or pigment, vehicle or binder, thinner, and drier or additive.
- Sequence on new plaster: allow the wall to dry (about six months), apply primer or cement primer, then two finishing coats; putty is applied before priming to fill undulations.
- On wood: knot the surface with shellac to stop resin bleeding, apply wood primer, then two coats of enamel.
- On steel: remove rust by wire brushing, apply red-oxide primer, then two coats of enamel or epoxy.
- Varnish types: French polish (shellac in spirit) for interior furniture, spar varnish for weather exposure, polyurethane for hard-wearing floors.
- Defects: chalking from underbound pigment, flaking from poor adhesion, blistering from trapped moisture, sagging from over-thinning.
Worked example
Paint for 120 m² of wall with two coats at a coverage of 10 m² per litre per coat. Paint per coat = 120 ÷ 10 = 12 litres; for two coats = 24 litres. Add 5% for wastage and touch-up, giving about 25 litres. Primer is estimated separately at one coat, roughly 12 litres plus wastage.
Traps that cost marks
Counting the primer as one of the two finishing coats; painting damp plaster and then blaming the paint; using French polish externally; forgetting that coverage falls on rough or sand-faced surfaces.