Preparation for Painting Old Walls
Wood primer is oily. Some of the oil is absorbed into the wood and helps to give good adhesion. Oily wood primer should not be thinned unless it is being used on highly porous wood. This should saturate the pores of the timber. If this does not happen, air will be trapped in the pores by the thick primer.
As the temperature rises, the air expands, causing bubbles to rise in the primer and burst through the paintwork. To avoid this on porous wood use a thin saturation coat of primer, then when dry apply a second primer coat.
An aluminium sealer is effective. It prevents seepage of resin in highly resinous timbers and problems such as creosote bleeding through the surface. The ends of window sills and thresholds should be stopped to seal the open grains. This should be done before undercoating.
Sound wood surfaces that need repainting still need preparation. The surface should be washed down and lightly glass-papered. Brush down to remove dust. Often a coat of primer, well applied, may last several paintings without being renewed. Exterior wooden surfaces that have cracks or indentations should be filled with hard-stopping after priming, then rubbed down before either undercoat or finish coats are applied.
Metal door and window frames and lead pipes need preparation. They are liable to be dirty, and coated with rust. Large areas should be scraped or brushed down with a wire brush and emery paper. It is important to protect the eyes when brushing down metal as particles of rust may cause discomfort and even damage.
Next, use rust solvent to remove the rust particles, microscopic in size, which will cause further trouble. In difficult-to-reach areas a proprietary rust curative, a preparation that turns rust to iron-oxide, can be used. This can, on, be painted over with bituminous-based paint. If the surface is good but smooth, it should be roughened to form a good key.
On window frames rust should be removed from metal with a wire brush. Rub down with emery paper and paint with a rust solvent. If the windows need reputtying, remove the old putty, treat the window frame for rust, prime the metal and then replace with metal glazing putty.
Walls can be protected against damp with cement or stone paints. Cement-based emulsion paints provide a damp-resistant layer but they are only as good as the wall beneath. This must also provide a waterproof barrier.
Before applying cement or stone paint the surface must be sound, dry and clean. If the rendering has blown (come away from the brickwork) it must be hacked back and re-rendered. Brush down the surface with a stiff brush and, if required, apply a clear sealant coat before covering with the decorative coats. Before applying an external wall cover coat, the wall should be quite dry.
Internally, surfaces that need preparation may be of wood, metal, plaster or paper on plaster. Plaster surfaces should be dry before painting. If painting a new wall, a water-based paint should be used to allow the plaster to dry out. Any cracks should be cut back and filled before painting or papering. Unless a papered surface is smooth it is advisable to remove the old paper before relining and repapering.
Wood surfaces, if sound, can be washed, rubbed down and painted. Badly pitted or blistered surfaces should be stripped back to the bare wood.
Where damage is only localized, treat the area separately-rubbing down and filling if necessary. If a complete change of colour is required, such as stripping a dark paint to repaint in a light colour, it is necessary to strip the paint back to the wood. Varnish should be removed before replacing with paint. It does not provide a good surface for new paint as adhesion, even after sanding down, is not good. Remove theand treat as new wood.
Polish, wax and silicone adhering to a surface can be removed with white-spirit. Rub off the wax or silicone with a rough cloth and sand down. If the paint separates, the surface is not clean enough. French polish can be removed with methylated spirits applied with a rough cloth.