KNOW
HOW
Paint Defects Most painting jobs are completed without any problems but occasionally things don’t go to plan. Here are some of the defects that may occur, their causes and how to remedy them.
BITTERNESS
A defect in which the appearance of a paint finish is marred by small particles of extraneous material, typically dust, grit, broken paint skin and fragments of bristle from brushes. To reinstate a bitty surface, allow the coating to harden and then rub it down carefully with fine waterproof abrasive paper, preferably used wet and recoat with fresh material using clean equipment. Remove skin and strain paint into a clean kettle. Remove all dust and dirt before painting and clean all brushes/rollers.
BLISTERING Essentially, blistering is a form of localised loss of adhesion of a flexible paint film. As the film becomes more rigid, e.g. with age, adhesion failure is more likely to take the form of flaking. Moisture beneath the paint film is the most common cause of blistering, especially on woodwork where it is almost inevitable if the moisture content of the wood is much in excess of 18 per cent for any length of time. Badly blistered paint must be stripped and, if moisture is the cause, time allowed for the substrate to dry out. Isolated blisters can usually be removed and the resulting depression filled and brought forward before recoating overall.
CISSING
This describes a defect in which freshly applied paint recedes from the surface leaving small craters or bare areas. The usual cause is contamination of the surface, e.g. by grease, oil, wax polish or silicones. Cissing may also occur when water-thinned paints are applied over glossy or semi-gloss oil based coatings. It can be prevented by ensuring that the surface is clean and by flatting oil based coatings before applying water-thinned materials. When cissing has occurred, the paint must be allowed to harden before it is rubbed down and recoated.
CURTAINING The usual causes are uneven application to broad, flat surfaces or over-application to mouldings and to rough or contoured surfaces. Failure to join up ‘wet edges’ before they have set may result in excessive film thickness and curtaining or sagging. To remedy the defect, the coating must be allowed to harden thoroughly before flatting down and recoating. Prevent curtaining by applying the coating evenly and avoid heavy coats. Plan the work accordingly so that the wet edge is kept open.
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