Paints, Coatings, Primers, Putties and Allied Materials
The exam tests whether you know what is inside a tin, which product belongs on which substrate, and how thinning, mixing and film thickness affect performance.
What paint is
Pigment gives colour, opacity and corrosion inhibition; the binder forms the film; the volatile thinner controls viscosity; additives include driers, anti-skin agents, biocides and thickeners. Push pigment volume concentration past the critical point and the film chalks, loses sealing power and fails early - cheap high-filler paint looks white but does not last.
Choosing the product
- Primers by substrate: wood primer for joinery, red oxide or zinc phosphate for steel, etch primer for galvanised sheet and aluminium, alkali-resisting primer for plaster.
- Finish coats: dry distemper (cheap, interior, not washable), washable distemper, cement paint (exterior masonry, water-cured), acrylic emulsion, synthetic enamel (wood and metal), two-pack PU or epoxy (hard, chemical resistant, finite pot life), varnish or lacquer for transparent wood.
- Coverage from the tin label: typically 8-10 sq m per litre per coat for emulsion, 12-14 for enamel.
Thinning, mixing, storage
Thin only as the label allows, often up to 5-10 percent for enamel; over-thinning destroys film build. Two-pack products: mix base and hardener in the stated ratio, allow induction time, finish within pot life. Store sealed at 5-30 degrees C, FIFO, and note the batch number for touch-up.
Worked example
A specification demands 40 microns dry film from a product with 50 percent volume solids. Wet film = 40 x 100 / 50 = 80 microns. If the painter lays on 50 microns wet, the dry film is only 25 microns - under specification, so opacity and durability fail inspection even though the surface looks painted.
Traps
Thinning by eye to make paint go further; using interior emulsion outside; mixing more two-pack than pot life allows; assuming coverage from tin size instead of the label.