Preparation - Priming & Sealing

New Plaster

Although new plaster can be primed with latex sealer, it is recommended that large plaster surfaces be primed with a properly formulated alkyd wall primer & sealer. Latex does not penetrate the plaster surface and if the plaster is powdery, over-trowelled, or if the plaster mix isn't quite right, poor adhesion of the latex can result. But, whichever you choose, alkyd or latex, the moisture content of the plaster must be below 14% prior to painting or failure can result.

Good Adhesion

When recoating a gloss or semigloss alkyd paint with the same finish, priming the old paint with an alkyd primer is not a good idea. Good painting practice dictates that the surface should be clean and deglossed by sanding prior to painting. The new alkyd finish coat will have better adhesion to the old coating than an alkyd primer will. Primers perform best on new or bare surfaces. You will, however, want to use an alkyd primer over an existing alkyd enamel, if the new finish coat is latex. The alkyd primer will improve the adhesion of the latex finish coat.

Liquid Sandpaper

Although sanding is the preferred choice to prepare a surface, liquid sandpapers are sold to improve adhesion when repainting an old, hard, glossy surface. These products are generally solvent blends that soften the paint surface for a short period. If used, the surface should be painted within 15-20 minutes of wiping down with liquid sandpaper, so only do small areas at a time. Liquid sandpapers are not very effective if the new finish coat is latex paint.

Textured Ceilings

Most texture sprays used on ceilings are sensitive to water but not sensitive to solvent. If the first time the texture is painted, a latex is used, the perimeter area cut in with a brush must be allowed to dry thoroughly before proceeding with rolling the remainder of the surface. If this is not done, and you roll into an area that has been softened by the water in the paint used to cut in, areas of texture may peel back to the primed drywall. This problem does not occur if you paint a textured ceiling the first time with alkyd flat. Future repaints can be done with alkyd or latex.

Asphalt Residue

Surfaces that have asphalt residues on them are difficult to paint. These surfaces include concrete floors where a residue of tile adhesive remains, or, an exterior wall coated with asphalt water proofing, that becomes an interior wall in a building addition. The asphalt will bleed through most solvent-based coatings. You can prevent bleeding by priming the surface with traffic paint or a shellac-based primer/sealer before the topcoat is applied. A spot test is recommended before proceeding.

Sanding

When sanding between coats of paint, the grit of the sandpaper is important. If the grit is #80 or less, it is possible to scratch the surface and the scratch marks will show through the next coat of paint. If the grit is too fine, the sandpaper will plug up. #120 grit is about right for sanding between coats of paint.

Steel / Aluminum / Vinyl Siding

When painting metal siding, use an exterior alkyd primer first - use the primer recommended for the topcoat you will be using. Depending on the factory finish used on the metal siding, it can be difficult to repaint unless it is somewhat weathered. The surface must be clean and free of "chalk" before priming. On vinyl siding, do a test patch for adhesion before proceeding, and then make sure that the color change is not too great. Dark colors absorb a lot of heat, and distortion or twisting of the vinyl siding can occur. Generally speaking it's wise to avoid going deeper in color than the original.

Galvanized Surfaces

Alkyd paints should not be applied directly on galvanized surfaces. Over time, alkyd paints can form acids which react with the zinc surface to form zinc soaps. Although the paint may have good adhesion initially, peeling usually results. Galvanized metal primers are formulated to prevent the formation of acids on aging. When investigating a peeling complaint on galvanized metal, one way to tell if a proper primer was used is to rub your finger on the side of the paint chip that was in contact with the galvanized surface and then smell your finger. The reaction product of the acid and zinc has a pungent odor.

Galvanized Metal

Galvanized metal panels that are used for ceilings and walls in commercial buildings are treated during the manufacturing process with chromates or oil to prevent the formation of "white rust". White rust is the stain that appears on galvanized panels when stored in a humid environment. These chromates or oil interfere with paint adhesion, especially if the exposure is going to be damp, in buildings such as arenas and swimming pools. The chromates should be removed by abrasive blasting and the oil removed with a solvent or detergent wash prior to painting. Vinyl wash primers in particular should not be applied over chromates.

Painting Over Vinyl Wallcovering

Rather than stripping 54' vinyl wallcovering in a commercial building, it is often painted when it becomes soiled or a color change is desired. If the surface is dirty, it should be washed with T.S.P. and then rinsed. If the topcoat is latex, it can be applied directly to the vinyl without a primer. If the topcoat is alkyd, a latex primer must be used first. Plasticisers in the vinyl can cause drying problems with alkyd paints if a latex primer is not used.

Before Installing Vinyl Wallcovering

Latex sealers are not recommended on drywall under vinyl wallcovering. Latex paint will not penetrate into the paper face of the drywall and tends to sit on the surface. Down the road, when the vinyl is stripped, it is likely that the paper face of the drywall will be damaged and/or peeled off. Alkyd paints, on the other hand, penetrate the paper face of the drywall, making it strong enough to withstand the stress of the vinyl being pulled from the surface.Traditionally, latex sealers are not recommended on drywall under vinyl wallcovering. Latex sealers do not provide ample barrier protection to the drywall surface from the wallcovering adhesive.  Therefore, the wallcovering adhesive could penetrate through the latex sealer and bond directly to the paper face of the drywall. Thus, when the vinyl is stripped, the paper face of the drywall will be damaged and/or peeled off. 

Alkyd paints, on the other hand, penetrate the paper face of the drywall and create a barrier to prevent the wallcovering adhesive from penetrating through it. Making it strong enough to withstand the stress of the vinyl being pulled from the surface. However, in recent years, wallcovering adhesive manufactures have reformulated their products to be higher solids and therefore less penetrating.  Some of the newer generations of wallcovering adhesives are not compatible with alkyd primers or paints.  Therefore, before installing any vinyl wallcovering check the adhesive manufacturer for the recommended primer type.

New Drywall

Before painting new drywall, check to see if the drywall was sanded with a 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, if the paper face of the drywall was roughed up, it will create a surface texture that is hard to get rid of. In many cases these defects will not be visible until the surface is primed. The defects must be repaired before the topcoat (especially alkyds) is applied.

Shellac

Shellac is often used as a sealer for knots in wood and as a fast dry clear finish. It is available in both an orange and white product. The orange is a better knot sealer than the white product. Shellac that is old should be checked to make sure it will still dry, because it loses its drying properties with age. Shellac should never be used under polyurethane varnishes since it will cause the polyurethane to peel, usually after a few months.

How To Remove Mildew

To remove mildew from a painted surface, wash the surface with the following mixture:

• 2/3 cup trisodium phosphate

• 1/3 cup detergent

• 1 quart chlorine bleach

• 3 quarts of warm water

Rinse the surface well with clean water after washing. Please wear appropriate safety equipment (rubber gloves and eye protection) and....protect shrubbery and plants with plastic sheeting.

Stains

If you have stains bleeding through a paint film, they can usually be stopped by switching the type of paint being used. Stains that are water soluble are generally not solvent soluble and stains that are solvent soluble are generally not water soluble. If in doubt, you can always use a "Brand Name" stain sealer.

Acoustical Surfaces

Acoustical surfaces that are designed for noise control, such as ceiling tiles, can be painted with flat paints without substantial reduction in acoustical properties. The use of eggshell, semigloss, and gloss paints, should be avoided because they have good sealing properties. The difference in acoustical properties between alkyd flat and latex flat is minimal.