Preparing Surfaces For Water Based Paints

Old Surfaces

Surfaces previously finished in water paint should not be washed down nor rubbed down wet, as this may weaken the adhesion. Wherever possible, the preparation should consist of dusting down, coupled where necessary, with dry rubbing down.

If the existing finish is in good condition the new paint may be applied directly upon it.

A stage is reached however, when the number of coats and the thickness of the film is so great as to affect adhesion to the initial coating even when the surface appears sound.

If any doubt exists as to the number or quality of the previous coatings, the work should be primed with an Oil based Sealer / Primer to bind down and increase stability.

On old coatings which are in bad condition, loose and flaking material must be scrapped off, the edges feathered with suitable filler, and the surface coated with an Oil Primer / Sealer.

Surfaces previously finished in size-bound distemper must be thoroughly washed off and sponged to remove all traces of powder and size. When dry, the surface is then primed with an Oil Primer / Sealer.

On surfaces painted with emulsion paints, where the existing paint is in good condition the water paint may be applied directly. If in bad condition, all loose materials should be scraped off and the surface bound down with an Oil Primer / Sealer.

Surfaces previously finished in oil paint are not suitable for decoration with water paint.

New Surfaces

Water based paint is eminently suitable for the decoration of new plaster, concrete, cement, etc.. Its tolerance for damp is such that even if the surface is not completely dry the moisture is not imprisoned by the paint film but is able to evaporate. This does not imply however, that it is safe to paint while the surface is wet; the drier the surface the firmer will be the adhesion. For actively alkaline surfaces, Sadolin Alkali Resisting Primer should be used prior to painting.

On surfaces to which water paint is normally applied, a special primer is not usually required. On completely dry plaster surfaces, a coat of alkali-resisting primer may be used to reduce porosity. Wallboards which are very absorbent, or the fibres of which might be raised by the action of water, should also be primed with an oil primer.

Efflorescence

This is a white crystalline deposit which forms on the face of plaster, brickwork, concrete, etc., due to soluble salts migrating from within the materials during drying out. If paint is applied before the efflorescence has ceased to form, the growth will push the paint off the surface. In the case of permeable materials such as water paint or emulsion paint the salts may penetrate the decoration and the crystals form on the face without causing damage. In general however, the safest course is to defer painting till all efflorescent activity has ended.

The treatment is to brush off the deposit, wipe the surface with a damp sponge and keep it under observation for a few days. If no further crystals are deposited it may be assumed that it is safe to decorate. The crystals should not be washed off as the introduction of water into the surface may renew and increase the growth. When efflorescence has occurred it is wise to apply an Alkali-Resisting Primer, but it should not be regarded as a cure for efflorescence.