Order of work.
The rule of exterior decorating is: Start at the top and work downwards. Start painting at the top of the house on the soffits and barge boards. Soffits are behind the gutters and barge boards may be below the gables of a tiled roof. Paint the top level guttering next, then the main facade, which may be rendering or cladding. Now paint the down-pipes, masking carefully any newly decorated surfaces. Next paint the window frames and sills. Paint the doors last.
There are basically seven materials used in painting external surfaces: Masonry paint; exterior-grade emulsion; wood primer; zinc chromate primer for the majority of metal surfaces; undercoat and top coat for alkyd-resin paints; knotting for sealing knots in new wood; and bituminous paint. Polyurethane paints are applied coat on coat.
Masonry paint is normally used on rendered surfaces. It both decorates and protects against damp. Some masonry paints dry to a smooth finish for easy cleaning. Others impart a textured surface to the rendering.
Exterior-grade emulsion can be used on timber or rendering. It has solely decorative properties. It will not provide a waterproof barrier. If using it as a rendering cover coat, apply a waterproof sealer before the decorative coat. Exterior-grade emulsion is also used to paint the exterior of asbestos guttering.
It is essential to seal new wood or wood that has been stripped back to prevent the wood from absorbing water. Wood primer provides this seal. Oil in the primer is absorbed into the wood to make the seal. Never use wood primer on a metal surface, for it does not inhibit rust.
Metal primer should be applied to exposed metal, other than lead or copper, to counteract the formation of rust. On steel, aluminium, weathered zinc or galvanized steel, use a zinc chromate primer. On new zinc or galvanized metal, use calciumplumbate primer.
Patent knotting seals the resinous knots in poorly seasoned, stripped or new wood. Sealing prevents the oil in the wood seeping out or “bleeding through” the decorative coats. Apply knotting before the priming coat.
Under and top coats.
Alkyd-resin paints are excellent for exterior use on timber, doors and window frames, iron gutterings and downpipes. For a really weatherproof surface use a gloss finish. One base coat and two top gloss coats will normally provide a durable finish. The undercoat, highly pigmented, fills in the wood and gives a solid base for the top coat.
Bituminous paint, highly water-resistant, is normally black but it is possible to obtain red or green. It is usually applied to the inside of metal or asbestos guttering to provide a water seal.
The tools and techniques involved in the pre-painting preparation of the exterior are in general similar to those used for the inside of the house.
Preparing metal frames.
Rusted metal on window or door frames should be stripped with a chemical paint stripper. If the metal is very badly rusted, use a wire brush to remove as much of the rust as possible. Rub down with emery paper and then apply a rust solvent. Where windows need reputtying, remove the old putty, wire-brush the inner frame, treat with rust solvent, prime and re-putty, using metal glazing mastic.
Faults include crazing, “chips” and blistering.
Crazing occurs when one coat of paint is applied over another that is entirely differently constituted. The two paint surfaces contract at different rates and crazing results. Treat small areas by rubbing down with wet-and-dry paper. Strip back large areas to the bare wood; then knot and prime before painting.
“Chips” or holing in the previously painted surface should be tackled in the following way. Rub down the old paint in the immediate area, “feathering” in the edges to avoid an obvious demarcation line. Fill the hole with hard-stopping. Allow this to set, then rub it down level with the surrounding surface.
Blistering is treated by lifting the affected area with a knife, priming the cavity, filling with hard-stopping and, once this is dry, rubbing down. Prime the repaired area.