Preparing New Surfaces

Plaster

Lime Plaster

Adequate time must be allowed for the structure to dry out and for the surface to cure.

If the surface is prematurely sealed with an impervious layer of oil paint, the moisture will force the paint off and the plaster may be prevented from hardening properly. If early decoration is necessary it should take the form of permeable material such as water paint or emulsion paint. When drying out is complete, an alkali-resisting primer should be applied before finishing with oil paint.

Gypsum Plaster

This must be allowed to dry thoroughly before being sealed with oil paint. The plaster should be chemically inert; alkaline matter may, however, be brought forward from the backing materials or lime may have been added by the plasterer. Before painting, test for alkalinity. If any doubt exists alkali-resisting primer must be used.

Concrete, Brickwork, Building Block and Slabs

All these materials are alkaline. Painting should be avoided until the dryingout process is complete. Ideally, the surface should be primed with alkali resisting primer. For commercial applications, emulsion paint may be applied directly as long as the surface has been allowed to dry.

Timber

Seasoned wood normally presents no difficulty. Dry timber rapidly absorbs moisture; priming should be carried out if possible before delivery to the site. Dry, un-primed timber on the site should be primed without delay. Prime all back edges which, when fixed, will be inaccessible. Double prime all end grain. Sandpaper the wood to remove dirt and dust off. Brush the primer well in and avoid fatty edges.

For most purposes the most suitable primer is white wood primer. For softwoods the primer should be oily enough to satisfy absorption and still bind the pigment particles left on the surface. For hardwoods the primer should be Aluminium Wood Primer thinned with up to ten per cent white spirit to assist penetration. Woods with considerable variations of absorption and woods with high resinous content (e.g. Colombian Pine) should be primed with Aluminium Wood Primer. Woods with high oil content e.g. teak, cedar, should be washed down with solvent before priming. Wood primers provide a good foundation for subsequent coats and promote overall film durability. Unseasoned wood is unsuitable for paint systems. However, if it cannot be avoided Sadolin ‘Pinotex’ PX 65 wood preservative should be applied.

Iron and Steel

These materials corrode rapidly, especially in chemical and coastal atmospheres. Rust itself contains the ingredients which cause further corrosion to take place. It is therefore essential to remove all rust before painting. New steelwork is often left to weather until the mill scale is loosened before painting is commenced.

Methods of cleaning include:-

  • Chipping and wire brushing by manual methods; However, this is tedious and not usually thorough enough.
  • Chipping and wire brushing with pneumatic equipment; While this is better on large areas than Manual methods it is likely to cause pitting of metal and to leave particles of rust untouched.
  • Sandblasting; normal sand blasting is unsuitable for site use but methods incorporating a vacuum process to reclaim the shots may be used. This method is very effective but expensive.
  • Acid pickling: very effective but not suitable for site use. Brush applied chemical solutions may be used on erected steel but may leave undesirable residues. The primers for iron and steel should be based on rust inhibitive pigments. These include red lead, zinc chromate, zinc rich (powdered zinc) etc. Particular attention should be paid to applying the primer around rivets and bolt heads and along edges.
Non Ferrous Metals

This includes newly galvanized iron. Remove dirt and grease. After cleaning, use Wash Primer or H.B Epoxy Primer. For Weathered galvanized iron, remove rust with wire brush, clean and remove grease. Then prime with Zinc Chromate Primer.

For Aluminium, after cleaning, prime with self etch Wash Primer. For metals containing copper and brass, etch with abrasive paper and white spirit, and apply a quick drying-sealer varnish pigmented with aluminium.

Asbestos Cement Sheets

Allow to dry out. Prime with alkali-resisting primer. If possible, paint the reverse side with bitumen paint before fixing.

Plasterboards

No special primer required.

Hardboards

Different brands vary in porosity. In some cases the oiliness of the material affects the drying of paint. Prime with Sadolin Acrylic Sealer Undercoat. Alternatively, prime with Aluminium Wood Primer.

Lining Paper, Canvas, Fabrics etc

Seal the fibres with a thin coat of Sadoprime Size before painting.

Acoustics Board

When boards are to be used for acoustics purposes they should not be sealed with hard-drying paints as this reduces their effectiveness.

Preparation of Old Work

Old paint should not be stripped off unless necessary. Paint needs stripping off if it is cracked, blistered, peeling, wrinkling, or affected by mould growth. When old paint does not need stripping it should be thoroughly prepared by rubbing down with pumice or abrasive paper and a solution of soap, and thoroughly rinsed off with clean water. Chalking paint should be well rubbed down to remove loose materials before repainting. Defects on plaster work should be made good.