Priming Interior Surfaces | |
Many building materials have surfaces with uneven porosity that will not take paint well without a preparatory coating. A single-wood board, for example, may have both hard and soft fiber layers; may have knots and open grain; and may have both open and closed grain in the same board. If you attempt to paint that board without priming it, the paint will be absorbed unequally into the wood, may lie unevenly on the surface, and may therefore have a mottled or varying sheen or gloss. For this reason most surfaces to be painted should have a first coating of primer, a specially formulated material that is applied directly over the bare wood (or other) surface. Primers contain pigments and fillers that help to even out any differences in surface texture and to present a uniform surface for applying the finish coat. Primers make the paint surface uniform and also provide a proper base to which the paint can adhere. Sealers are formulated with a high resin content to create a film or barrier so that water or paint solvent cannot penetrate the surface. Sealers equalize the porosity or suction of the surface to be painted, so that the finish coat will lie evenly as a protective coating on the surface of the workpiece and will not soak into the material. This ensures a better appearance, because painting a surface with uneven suction or porosity may cause both the color and the gloss level or sheen of the finish coat to appear uneven or patchy. When redecorating an older house that has no vapor barrier in the exterior walls and/or ceilings, one can establish a vapor barrier by applying a coat of oil-base primer over the surfaces in question. This oil-base primer provides a suitable film to reduce moisture transmission through the wall or ceiling. Be sure to remove any electrical outlet covers or heating duct registers before priming to ensure complete coverage. If there are cracks between the outlets and the plaster or wallboard, fill the cracks with spackle before priming the wall to make the vapor barrier as complete as possible. In most cases, any sound, existing paint finish will serve as a base or "primer" for repainting walls or ceilings. If these surfaces have been widely repaired, apply a coat of primer over the entire wall or ceiling before painting. Spot priming is possible for small repair areas, but keep in mind that these well-primed spots may look "shiny" when repainted and compared to the unprimed areas of the wall or ceiling. If one plans to apply wallcovering to the surface (wall or ceiling), applying an alkyd primer over the entire area is recommended to provide a tight seal and to ensure easy removal of the wallcovering in the future. Priming Textured Ceilings Along about the mid-'50s, the ceiling finish of choice became some sort of texture, either hand or machine (spray) applied. Most of these texture materials were extremely soft and provided no seal for easy cleaning or for repainting. As a result these unprimed ceilings are almost impossible to repaint, because their soft, absorbent surface will take paint unevenly. Lap or roller marks plus uneven paint film result if you apply any ceiling paint over an unprimed texture paint surface. To prevent this uneven application you must apply a coat of alkyd primer over the texture, then finish with the topcoat of choice. We suggest that you buy a quality latex paint and roll it over the primed texture ceiling. Choose a latex paint designated "Ceiling Flat White," or one with a very flat sheen. Any gloss on the finish coat will highlight ceiling defects: the flatter the better, when it comes to the gloss of any paint, and it is especially important for ceilings. Ceilings are the largest unbroken surfaces in a house -- unbroken by openings such as windows and doors, or by furniture placement -- so one must choose a ceiling paint that has very low gloss, and thus very little light reflection, to have a flattering ceiling finish. Priming For Smoke, Difficult Stains Smoke stains from either tobacco or from a house fire are very difficult to cover. Nicotine in the tobacco smoke and grease or oil in the house fire smoke will bleed through latex paints, because the water in latex paints will activate the staining agent. The same thing is true of rust stains that damage plaster or wallboard from leaking radiators or hot water pipes. In most cases one can clean smoke from the wall or ceiling surface using a cleaner such as TSP (trisodium phosphate.) Remove as much of the smoke as possible. For limited smoke damage, apply a coat of alkyd primer over the affected surface. For very difficult stains a fast-drying shellac sealer such as BIN will be a better choice. BIN will seal over smoke, oil, rust, crayon or markers. If you use a shellac-base sealer such as BIN you must use alcohol as a solvent to clean up spills, spatters, and tools. Priming New Wallboard Wallboard presents a perfect example of why we need primers and sealers. Newly finished wallboard presents two distinctly different surfaces to be decorated: the face paper of the wallboard over the field or untreated area of the wallboard, and the taping compound over the seams, corners, and nails or screws. Paint applied to the face paper and the compound will be absorbed unequally and will exhibit problems such as "joint banding," a phenomenon that makes the paint appear shadowed, or darker, over the areas treated with compound. Some texts recommend using oil-base primers over wallboard, but the Gypsum Association and its member firms have never recommended this practice. Oil-base primers are slow drying and tend to soak into and raise the nap on the face paper of the wallboard. But the oil-base primer will dry smooth over areas that are treated with taping compound. The result is a mess, with a blotchy surface that is alternately rough and smooth. The only (new) wallboard surface that should be primed with oil-base or alkyd primer is any surface that will be finished with a texture paint, such as ceilings that will be spray-textured. For any other surface, the wallboard industry advises using a heavy-bodied latex paint as a first or prime coat. For years the paint industry offered "primer-sealers" for first-coat application on wallboard, but there are distinct differences between primers and sealers. The problem is that many primers do not contain the proper type and amount of pigments and fillers to conceal the surface texture differences on wallboard. By the same token, sealers may have a high resin content to provide a film that prevents penetration of water or paint solvent, so it equalizes the porosity of the wallboard surfaces, but it cannot eliminate the surface texture differences of the wallboard. Neither primers nor sealers are recommended as a first coat on gypsum wallboard. Use an undiluted interior flat latex wall paint for the first or prime coat on wallboard. When this first coat is dry you can then apply a finish coat of any sheen or gloss paint finish. Our favorite choice as a first coat on wallboard is a latex primer named, oddly enough, Sheetrock(TM) First Coat. A product of the United States Gypsum Co. (USG), First Coat is available either in powder form, to be mixed with water, or factory mixed in paint form. The product is fast-drying for quicker recoating, seals the surface to equalize paint absorption, and minimizes surface texture variations. Ask your dealer to order First Coat for you if you don't see it in stock. Note that the USG, the inventor of Sheetrock(tm) and a major manufacturer of wallboard and wallboard products, does not suggest oil-base primers as a first or primer coat on walls or ceilings that will be decorated with wallcoverings. Rather, USG advises that a prime application of First Coat should be followed with a coat of sizing prior to hanging wallcovering. Clear Wood Sealers The time-honored method of finishing woodwork or furniture to have a natural wood finish was to apply a clear sealer, then apply a polyurethane or varnish finish as a topcoat. Or, a stain was applied to give the wood a preferred tone or color. The stain was followed by sealer (sometimes called a "sanding sealer") to minimize the absorption across the surface of the wood. The sealer would raise the grain of the wood, so sanding was necessary before the finish coat of varnish or other finish was applied. Today, some wood finishes may combine sealer and finish, or even include the stain, so that a wipe-on product provides acceptable results with minimum effort and skill. However, any finish product will tend to raise the grain of the wood, so you should sand between coats using a fine sandpaper. This sanding between coats will also remove any airborne lint or dust that has settled on the wet finish, and ensure that your finish project is smooth and blemish-free. Conclusion Basic painting wisdom tells us that any surface to be repainted should be clean, dry, and free from peeling or excessive chalking. Beyond these basic and rather obvious criteria, when should one use a primer or a sealer? For best results, one should always use a primer as a first coat on new materials. Also, apply a primer on any surface that has been extensively repaired, such as a cracked plaster wall or ceiling, to equalize the suction between the new repair areas and the old plaster. If a house interior has been damaged by smoke or fire, first clean the smoke residue away, using TSP or an equally strong cleaner. Then prime the smoke-damaged areas to seal in the smoke and prevent it from being activated and bleeding through the new paint finish. For sealing over difficult stains such as smoke, grease or oil, a shellac sealer is recommended. If the budget is a concern but you feel the surface needs priming, you can have the paint dealer tint the primer slightly to match the color of the finish paint coat, so you can get by with a two-coat job. For priming on wallboard interiors, pro painters often consider a heavy-bodied paint as being "self priming," so the finish coat is limited to two coats of latex paint. The basic rule is to always follow the paint manufacturers' label instructions when painting, and when in doubt, use a primer or sealer. written by Gary Branson Reprinted with permission. Copyright HouseNet, Inc. |