Good preparation essential for a lasting paint job

Press, Volume CXV, Issue 33987, 30 October 1975, Page 11

Good preparation essential for a lasting paint job

Last month this page dealt with the two different types of paint—enamel and plastic—but little space was devoted to a very important aspect of painting—preparation.

Paint manufacturers stress that the best paint in the world will not be a success if the proper preparation has not been done beforehand, says it pays to spend time and effort in getting the surface to be painted ready before a □rush is used. The technical committee of the New Zealand Paint Manufacturers’ Association have prepared instructions for painting exterior woodwork of houses or other buildings. Without the protection of a surface coating, timber weatherboards are progressively ravaged by the elements. Paint is the most widely used protection, all over the world. It has proved itself an economical, effective and versatile form of surface protection and offers the widest possible choice of colour. But paint needs to be applied correctly if full advantage is to be taken of its protective and decorative qualities. The procedures are simple and straightforward. If followed, they assure a lasting and attractive job. Weatherboard preparation can involve: new weatherboards for priming, primed, erected weatherboards, painted weatherboards for repainting. Procedures vary but all aim at preparing the surface adequately to take a uniform, tightly adherent paint coating of the correct thickness. That combination is the secret of successful painting. The weatherboarding should be smooth, clean and free from raised or woolly grain or other surface defects. There should be no sharp, exposed corners. Remove unwanted grain, corners and dirt by sanding with a medium grade abrasive paper (about 80 grit). The more thoroughly.you do this, the better the finish. Dust off carefully. If the moisture content is more than 18 per cent, adhesion of the primer will

be impaired. The only practical method of checking is with an electrical moisture meter. In practice, good housekeeping will prevent boards becoming too wet to paint. Cover weatherboard stocks with plastic sheeting or something similar. Do not work on the boards in obviously damp conditions, or when rain is likely. Discard poor quality boards containing loose knots, gum, bark streaks, or crossed grain. They will create problems ' later. Prime the laps and end grains thoroughly before erecting. PRIMED TIMBER Check primed boards carefully to assess surface condition. Overthinning of the primer may have resulted in too low a film build up, and too much penetration of the primer vehicle into the timber, leaving an under-bound film on the surface. Poor brushing techniques may have caused low or uneven film build up. When left exposed, oilbased primers are generally less durable than the synthetic vehicle types. If correctly applied, a good quality oil-based primer retains its protective qualities for about four weeks, while a synthetic primer may be satisfactory for slightly longer. Primed boards may have been left unpainted longer than a month or so. Overexposure of the primer will have broken down its essential adhesion and elasticity. If painted over in this condition, the life of the whole paint system will be reduced significantly. During examination, take the opportunity to stop nail holes and other imperfections with linseed oil putty. Any primed surfaces which have chalked should be sanded back with a medium grade abrasive paper

(.about 80 grit) until sound primer is reached, or the timber surface is exposed. All bare timber and thin areas of primer must be reprimed. Dust off thoroughly before applying further priming or finishing paints. PAINTED BOARDS Repainting over an unsound paint surface never gives satisfactory results. Frequently, the old, residual coating will flake off after only one or two years, taking the new paint with it. It is thus essential to assess carefully the state of the existing paint coats and remove undesirable surface deposits and unsound paint. Resist the temptation to clean up the obvious areas only. Quite often the surrounding areas are also affected, and need attention. Ageing and exterior exposure result in four major changes occurring in paint films: Chalking on all weathered surfaces. Formation of chemical •‘breakdown” products, which are not usually washed off surfaces protected by sheltered porches or overhanging eaves. Embrittlement due to continuing oxidation reactions, similar to those which occur during drying. As a result, the film can no longer expand and contract with the timber. Old paint coats sometimes lose adhesion to one another or to the timber surface. Under damp or humid conditions, fungal, moss or lichen growths may develop. The detection of unsound areas is not always easy. The following hints will be found helpful. To recognise chalking, rub the surface with a small pad of contrasting coloured rag. Examine the pad for a white (or coloured) pigment stain.

This loose layer of chalk normally affects old paint and is not necessarily a cause for repainting, unless it is very dense and the paint film has eroded away to a substantial degree. Chemical ‘'breakdown’’ products on sheltered sections of a building are not easily recognised.

They mat' show up as a slight scum along with general dust and dirt. However, it should always be assumed they are present on aged paint film.

Poor adhesion of older coats of paint to each other or to the underlying weatherboard is ’ not usually obvious but can be demonstrated by a simple procedure called the tape test.

Clean a small area with a cloth to remove chalk and dirt. Press a 2 or 3in. strip of adhesive tape firmly to the clean patch, leaving an end turned for gripping. Tear off the tape with a quick pull at right angles to the surface. If the paint coat is removed with the tape it is a sure sign of poor adhesion, and the old coating should be removed before painting.

Carry out the test on two or three widely spaced test areas on each wall, or on each type of surface to be painted.

Cracking, peeling, blistering and flaking are all obvious defects, and even in their early stages can be easily seen. However, to ensure the breakdown is not spreading, apply the tape test to all suspect areas. SURFACE PREPARATION Good surface preparation is essential. Extra attention, at this stage can more than double the life of the new paint system. Areas of sound paint showing chalkiness, surface scum or dirt can be cleaned with a brush or rough sacking pad and running water.

A mild soap or detergent solution may be used, if necessary', but the suds must be well flushed away with clean running water. Brushing and washing will normally remove chalk, dirt, scum, light moss or mould and even some of the breakdown products. A wipe over with mineral turpentine on a clean cloth, followed by a light sanding with medium abrasive paper will complete the cleaning of sheltered areas.

All areas of unsound paint (cracking, peeling, blistering, flaking, poorly'

adhering) should be completely stripped by burning off, power disc sanding, or by using a solvent based stripper (but not an alkaline stripper).

Burning off weatherboards is a skilled and hazardous process which should only be carried out by a competent tradesman, or under his constant supervision.

After burning off. the timber should be sanded down in preparation for immediate priming. Power disc sanding is a much safer process. Some scouring of the boards will occur, but this can be removed by hand sanding along the grain with fine grit. Remember that all sol-; vent strippers are injurious! to the skin and are toxic, j Take precautions to pre-, vent skin contact with the solvents and inhalation of the vapours. Some strippers contain wax, and care must be taken to remove all residues from the surface, inf accordance with the manufacturers’ instructions. Although useful, particularly on small areas, strippers are usually impracti-' cal and too expensive! where large areas of; weatherboard are involved. Where maintenance of j the old paint film has been; neglected and the wood' surface has been exposed.;' raised grain and soft sur-i face wood fibres must be; thoroughly removed, by scraping, planing or sanding.. Where boards are cracked or sun checked, puttying or filling will be necessary. This is feasible only where the defect is slight. Where timber cracking is severe, the only satisfactory remedy is to replace the weatherboard.

,• W'here peeling, cracking etc are localised in an’ otherwise sound area, they; are probably due to acci-i dental mechanical damage,! rather than weathering. The areas should be scraped and sanded to a firm, smooth, feather edge l of the sound paint, then primed immediately. In general, priming of'all! exposed, prepared wood! should be carried out the. same day as the prepara-' tion is done.