Although new plaster can be primed with latex sealer, it's recommended that large plaster surfaces be primed with a properly formulated oil-base wall primer-sealer. This is partly because latex does not penetrate the plaster surface and if the plaster is powdery, over-trowelled, or its mix isn't quite right, poor paint adhesion can result. Keep in mind that regardless of whether you choose an oil or latex, the moisture content of the plaster must be below 14 percent for paint to be successfully applied.
When recoating a gloss or semi-gloss oil paint with the same finish, do not prime the old paint using an oil-base primer. Instead, clean and sand the surface to de-gloss it. The new oil topcoat will then have better adhesion to the old coating - better than an oil primer. Primers perform best on new or bare surfaces. You will, however, want to use an oil-base primer over existing oil-base enamel if the topcoat is latex. The oil primer will improve adhesion of the latex finish coat.
When sanding between coats of paint, keep in mind that a grit of 80 or less may scratch the surface to the point where scratch marks will show through on the next coat of paint. If the grit is too fine, the sandpaper will plug up. A 120 grit is usually about right.
Before painting new drywall, check to see if it was sanded with sandpaper that was too coarse, leaving scratch marks in the taping compound or cutting through the paper face of the drywall board near joints or patches. The scratch marks will show through several coats of paint, and the roughed up paper face will create an uneven surface texture. In many cases these defects must be repaired before the topcoat (especially an oil-base paint) is applied.
Depending on the factory finish used on steel, aluminum or vinyl siding, it can be difficult to repaint unless it's somewhat weathered. The surface must be clean and free of chalk before priming. On vinyl siding, do a test patch for adhesion before proceeding. Also make sure that the colour change is not too great. Dark colours absorb a lot of heat, and distortion or twisting of the vinyl can occur. Generally speaking, avoid going deeper than the original colour.
The maximum moisture content for any surface to be painted is 12 to 14 percent. Painting a surface that contains a higher level of moisture can lead to peeling. The only accurate way to determine moisture content is to use an electronic meter.
There is a limit to the number of coats of paint that a surface can support. As the paint thickness builds up over time, and the coatings age, they lose their flexibility and can no longer expand and contract with the surface they're on. Cracking or flaking usually results. It is not uncommon to put a coat of paint on a surface that appears sound, only to have many layers of paint peel away. At this point the surface must be stripped.
There are two reasons for sanding wood prior to staining. The most obvious is making the wood smooth, but another is increasing the porosity of the wood, which makes the penetration of the stain more uniform - especially important with transparent stains.
Most paints on varnish adhere poorly and chip easily. Although de-glossing by sanding with 120 grit sandpaper can help, for best results, strip off the varnish with paint remover and then prime with an oil-base primer before covering with an oil or latex topcoat.
Pressure-treated lumber is kiln-dried wood (usually spruce or yellow pine) that is treated with chemical preservatives dispersed in water. To check if the wood is dry enough for a stain or paint, put water drops on the wood's surface. If the water soaks in, the wood is ready. It can then be painted or stained like any other wood surface. Over time, pressure-treated wood that is not painted or stained will weather, crack and check as badly as untreated wood.
Wood that has been exposed to the sun for more than a month should be sanded before painting, especially if the wood has started to turn grey. Sunlight causes the wood fibres to chalk and erode unevenly. They then form a weak surface layer that has poor adhesion and can cause the paint to peel. To test for sun damage, press a piece of tape on the surface, rub the back of the tape to ensure good contact, and then strip off the tape. If a lot of wood fibres cling to the tape, the wood needs sanding before painting.
Moisture in plywood causes the top veneer to crack. To prevent this and properly seal the wood and prepare it for a latex paint or stain, apply two coats of exterior oil-base or high-quality acrylic primer first. Never use semi-transparent stain on plywood; it does not provide enough protection from moisture, allowing the plywood to crack and check.
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