Paper is a mat composed mainly of cellulose fibres derived from timber. The fibres may be derived from either a chemical or mechanical pulping process; may be bleached or unbleached; with or without resin additives for wet-strength; and with or without surface treatments such as coatings or machine colendarings. Paper, for the most part, presents an excellent surface for painting; and water-based paints are pre-eminent in this area. They are normally applied direct to paper. There are however some areas where the use of an oil-based sealer, such as Resene Sureseal, is advisable:
1) Yellowing of unbleached paper - Unbleached paper in the presence of Ultra Violet light with yellow due to the formation of dye-like substances. These can cause discoloration of the first coat of a water-based paint applied directly to the paper. Use of an oil-based sealer obviates this.
2) Raised Fibres - Where fibres are raised - either from the type of paper or from sanding adjacent filler - water-based paints will aggevate the problem and set the fibres in position. Treatment with an oil-based sealer will allow successful subsequent sanding of the paper.
3) Old wallpaper
Wallpaper should be checked prior to painting to determine the condition of the coating used. Some old wallpapers used a weak coating which provides an unsatisfactory base for painting. If the old coating will come off as a powder when scratched with a fingernail, an oil-based sealer will condition the surface successfully.
Wallpaper printed with metallic inks should have a test area done to ensure there is no reaction with paints. Any sign of staining would indicate that a sealing operation is necessary.
Apart from the above comments, surface preparation of papers is straight forward; simply ensure that the surface is clean and free from dust, dirt, grease and mould. Cleaning methods should be with soft cloths as abrasives will damage the paper surface. Mould should be treated with Resene Moss and Mould Killer.
Gypsum plaster is the product of mixing Gypsum (calcium sulphate hemihydrate) with water. The gypsum takes up water and sets forming the di-hydrate. The set plaster is relatively weak compared to cement renders and always requires a penetrating surface conditioner such as Resene Sureseal. Gypsum plaster is slowly soluble in water and must not be used in wet areas. Pure gypsum plaster is chemically inert but impurities such as sodium sulphate can cause efflorescence; and additives such as lime putty can cause the surface to be alkaline. If gypsum is deprived of water before setting, e.g. thin fiIms over porous surfaces, or thin films on a hot day, setting will not take place. A powdery layer will be left on the surface. Fibrous Plaster is cast from moulds normally using a tallow-based release agent; residues of this can cause painting problems. Where tallow residues are suspected (usually on highly glazes areas), wipe the surface with a turps rag before painting.
Ensure plaster is thoroughly dry before painting;
Remove efflorescence with dry soft brushes or cloths;
Sand off imperfections and remove dust;
Solvent wipe any highly glazed area;
Pre-treat with one coat Resene Sureseal.
The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.