Paints, Varnishes, Lacquers and Thinners: Composition and Selection
Paints, Varnishes, Lacquers and Thinners: Composition and Selection
This domain tests the four parts of a paint, the differences between major product families, and how thinners and hardeners are chosen. Selection questions usually give a service condition and ask for the suitable product.
Composition
- Binder or resin forms the film and controls adhesion, flexibility and chemical resistance.
- Pigment gives colour, opacity and corrosion inhibition; extender gives body and sanding properties.
- Solvent or water controls viscosity and application; it evaporates and is not part of the dry film.
- Additives control flow, skinning, mould, settling and drying.
Product families
- Alkyd enamel: solvent-borne, easy brush application, good gloss, limited alkali and chemical resistance.
- Acrylic emulsion: water-borne, quick drying, low odour, good exterior colour retention.
- Epoxy: two-pack, excellent adhesion and chemical resistance, chalky in sunlight, so needs a topcoat.
- Polyurethane: two-pack, tough and gloss-retentive; aliphatic grades resist UV yellowing.
- Nitrocellulose lacquer: fast-drying, thin film, used on wood; needs a compatible lacquer thinner.
- Varnish: clear binder with no pigment; interior or exterior grades differ in UV resistance.
Worked example
An alkyd gloss is too thick for brushing. The correct thinner is white spirit at about 5–10% by volume, added gradually. Cellulose thinner is wrong because it is a strong solvent for nitrocellulose and will shock the alkyd, causing seeding or poor flow.
Traps
- Over-thinning a two-pack epoxy beyond the maker limit and reducing sag resistance.
- Mixing a two-pack product and using it after pot life; it will not cure properly.
- Using the wrong thinner for a lacquer and causing blushing.
- Assuming a clear varnish gives UV protection; many do not.