- 2064 Jestha (old course) · 16 marks
Paints, varnishes, painting procedures
Short answer
Paint: mixture of pigment, vehicle, thinner forms opaque protective film. Varnish: resin in solvent, hard transparent gloss. Oil paint constituents: base (opacity), vehicle (linseed oil), pigment, drier, thinner, extender. New wood: knot, prime, putty, undercoat, finish; old: strip, spot prime; metal: rust removal, red oxide primer; cement: dry plaster, alkali primer, two coats (cement paint cured).
Define paint and varnishes used in the building. Illustrate the constituents of an oil paint. Describe the painting procedures on; new and old woodworks, metal works and cement surfaces.
Paint and varnish
- Paint: a mixture of pigment, vehicle and thinner that forms an opaque, coloured, protective film on a surface when dry.
- Varnish: a solution of resin (lac, copal, rosin) in a solvent (turpentine, spirit or oil) which dries to a hard, glossy, transparent film. It protects woodwork and brings out its natural grain. Types: oil varnish, spirit varnish, turpentine varnish, water varnish.
Constituents of oil paint
| Ingredient | Example | Function |
|---|---|---|
| Base | White lead, zinc white, titanium dioxide, red oxide | Gives body, opacity and covering power |
| Vehicle (carrier) | Linseed oil, tung oil, poppy-seed oil | Carries the pigment, spreads and dries to a hard film |
| Pigment | Iron oxide (red), chrome (yellow), carbon black, ultramarine | Gives colour |
| Drier | Litharge, red lead, manganese salts | Speeds up drying of the oil |
| Thinner (solvent) | Turpentine, mineral spirit | Makes the paint workable; evaporates after application |
| Extender (adulterant) | Barytes, gypsum, china clay | Adds bulk, reduces cost |
Painting procedures
1. New woodwork
- Rub the surface with sandpaper; seal knots with knotting (shellac).
- Apply a priming coat (red lead with linseed oil); allow 24 hours to dry.
- Fill holes and cracks with putty; sand smooth.
- Apply one or two undercoats, sandpapering each after it dries.
- Apply one or two finishing coats of the final colour.
2. Old woodwork
- Remove old peeling paint by scraping, a blow lamp or paint remover; wash and dry.
- Rub down with sandpaper and fill defects with putty.
- Spot-prime bare areas.
- Apply undercoat and then finishing coats as for new work.
3. Metal work
Remove rust and scale with a wire brush or sandpaper; apply a red oxide or zinc chromate primer; apply an undercoat after 24 hours; finish with one or two coats of enamel or synthetic paint.
4. Cement/plastered surfaces
The plaster must be dry (3-4 weeks old). Remove dust and efflorescence, apply an alkali-resistant primer, fill defects with wall putty, and apply two or more coats of emulsion, distemper or cement paint (wetting the surface before cement paint and curing it afterwards).
✓ Answer checked by Claude Opus 5.5 (high effort)
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