Don’t be put off that painting job

Press, 4 March 1989, Page 16

Don’t be put off that painting job

Home Line with

Bill Harrison

Has your excuse for putting off the repainting jobs around the outside of the house this summer been an uncertainty about which type of exterior paint to use, oil-based or water-based? Well, the cooler autumn days provide ideal painting conditions, and there is sound information available to help you choose the right paints. Since the banning of lead pigments from paints for health reasons 10 years ago the choice has been either oil-alkyds or water-based acrylics. The question is, of course, which to use?

The table showing comparisons of the properties of alkyd and acrylic exterior paints was produced by a Building Reseach Association scientist as an impartial quide to decision-making.

Item 5 recommends the use of oil-alkyds on windows and doors to avoid the tendency of acrylic surfaces to stick’together. Item 12 cautions against coating over acrylic with oil-alkyd paints. If you are unsure what type of paint has been used previously on your house, try rubbing it with methylated spirit. This will cause an alkyd paint to shine, and an acrylic to wear away.

A simple test for the presence of lead in paints used, previously can be made with a diluted solution of sodium sulphide (5 per cent by weight) in water. A drop of the solution on the exposed layers of paint will turn a layer black if lead is present. A dust mask costing only a few cents should be worn as essential protection when sanding down old paint work. Even if the top coats don’t contain lead it is quite likely the primer coat does. Preparing a surface for repainting by washing with detergent, rinsing, and sanding to remove excess build up or remaining glossy areas is a common recommendation.

Paint manufacturers will usually provide technical advice and a pro-

duct specification for a particular problem or paint failure.

Burning off the old paint is definitely not an option for the inexperienced do-it-yourselfer. If such hazardous work is unavoidable it should be left to a professional with lots of exxperience and even greater amounts of insurance.

COMPARISON OF PROPERTIES OF OIL-ALKYD AND ACRYLIC PAINTS

OIL ALKYDS (exterior) 1. High initial gloss (if required) which decreases on weathering. 2. Adhesion to damp surfaces is poor; good adhesion to dry surfaces. 3. Resin particles in oil-alkyds are relatively small and can penetrate and bind lightly chalked and poorly prepared surfaces. 4. Paint flows out well while it slowly dries; wet edges last longer to allow good lapping. 5. Good “blocking” resistance. (Dry paint surfaces do not stick together). Useful on joinery. 6. Poor chemical resistance to alkaline surfaces of cement based materials unless carefully formulated. 7. Sanding of old paint is easier, burning-off also easier because the paint bubbles when heated. 8. Cleaning up is messier and less convenient; flammable, odorous solvents (sometimes moderately toxic). 9. Jobs can take longer since drying between coats is longer. 10. Hard surface with good stain and abrasion resistance. 11. Not very permeable to water. 12. Can be recoated with either oilalkyd or acrylic paints. 13. Eventually becomes brittle and cracks. 14. Surface of paint eroded by weather causing chalking.. 15. More tolerant of some cool or wet periods during drying. 16. Grain raising of timber delayed. 17. Compatible with both oil-based and water-thinned fillers. 18. Higher solids content makes it easier to achieve a normal dry film build. 19. Tendency to yellow. 20. Separate undercoat and topcoat.

ACRYLIC PAINTS (exterior) Initial gloss is lower, but good retention. Adhesion to slightly damp surfaces less affected provided normal drying follows. Resin particles in latex polymers are larger and have less ability to penetrate and bind. Paint flow is poor if drying is too fast, as on warm windy days; wet edges do not last as long. Poor “blocking” resistance and not generally used on the fitting surfaces of doors or windows. Good chemical resistance to cement based materials. Well suited to painting concrete and fibre-cement boards. Plastic nature of paint blocks up sandpaper. Burning off is difficult since the paint melts when heated — unless over oil based paint. Cleaning up is cheap and simple using water; non-flammable, little odor and low toxicity. Jobs can be quicker when several coats are needed. Softer suface has poorer stain and abrasion resistance. Generally more permeable to water. Durability of paint work after recoating with oil-alkyds unknown. Much more flexible, slower to become brittle. Chalking is slower. Should not be used if less than 10 degrees C. during application and drying. Poor early shower resistance. Grain raising of timber occurs immediately (with relief of surface stresses). Poor wetting of oil-based putties. Lower solids content requires that more paint be applied. Non-yellowing. Same paint is used for both.