How to Fill Cracks for Paint Preparation

Knotting:

This is a prepared shellac dissolved in methylated spirits. Knotting is obtainable in small quantities from local paint shops. It is used mainly to seal knots in new or bared wood before it is primed. The protective seal formed by brushing knotting over the knot, and a small area surrounding the knot, prevents resin exuding from the wood and creeping through the new paint. Knotting may also be used as a sealer for other purposes and this use will be explained later in these instructions.

Fillers:

Fillers, as their name implies, are used for filling small holes and cracks in wood. The process of filling large holes is known as ‘stopping’. In interior decorating the most common type of filler or stopping used in preparing surfaces, is glazier’s putty. This is obtainable in small quantities from local paint shops and is best purchased in sealed tins rather than loose quantities wrapped in paper. Putty is a mixture of whitening and linseed oil which hardens on exposure to air. When putty is not in use, the container should be kept sealed or the putty will harden in the tin.

Before stopping holes and cracks with putty — this is usually done after priming new wood, or undercoating old surfaces — the inside surfaces of the crack or hole should be coated with paint. If this is not done the bare wood will absorb the linseed oil in the putty and the fresh stopping will shrink and fall out. In addition to putty there are several forms of patent fillers and stoppers, but these are used mainly for preparing furniture before staining and polishing. However, some of them may be used when preparing interior surfaces. The most common of these is plastic wood, which hardens rapidly after exposure to air.

The plastic wood is pressed into the crack or hole with a putty knife and is modelled slightly proud of the surface – this simply means to stand above the level of the surrounding surface. The plastic wood shrinks slightly as it dries and after drying, any surplus stopping, still proud of the surface, is cut down with glass-paper. If the repair is a large one it will be necessary to apply this type of filler in several stages. If this is not done the exposed surface of the plastic wood will harden to form a skin over the still soft core of the stopping, which may take a considerable time to harden.

For most purposes putty is the most suitable and economic form of stopping, especially with interior work. However, there is a variation of putty; this is a patent, finely-powdered plaster-like material used for stopping holes and cracks in walls and ceilings. This powder when mixed with water may also be used successfully for filling holes and cracks in wood surfaces, and it will be found especially useful for preparing large flat surfaces, such as the face sides of flush doors. The powder filler is mixed according to container instructions and is then applied with a putty knife or with the edge of a broad stripping knife.

It is then left to harden, which it does very quickly, before sanding down. There is one important thing to appreciate about these fillers, although they are satisfactory for stopping wood surfaces; the hardened filler may not be quite as hard as the surface on which it is used, and if the sanding is done too vigorously, this will cut the filler below the decorating surface and the blemish will show through the filled job.

All the preparation in interior decorating should be done thoroughly. Success in finishing with a professional surface is not due entirely to the way in which coats of paint are applied. The quality of the surface finish depends on the thoroughness of preparation.

24. July 2011 by admin
Categories: Painting, Preparation | Tags: , | Comments Off on How to Fill Cracks for Paint Preparation

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