Guide to Working with Plastics: A Wealth of Fillers and Paints--Preparing a Compatible Surface for Paint

Plastic-base paints, no matter how superior their credentials, will perform well only on a surface that has been properly prepared. The surface must provide a strong base for the new coating. If it has weak spots or is rough or uneven, follow the steps to reinforce, smooth and fill it. After this stage, preparation varies with the type of surface and whether it’s already coated.

If the surface has been previously painted or sealed, first make sure that the coating you are planning to apply is compatible with the old coating (chart). Once you know that the coatings are compatible, roughen any glossy areas of the old paint with sandpaper, steel wool or a commercial de-glossing solution, obtainable at paint stores. Flaking or peeling paint must be taken off completely; hand scrapers, chemical paint removers and heat guns are good for this job. Large exterior metal and masonry surfaces, such as wrought-iron fencing and brick walls, can be cleaned with power tools—sandblasters for met al, water-blasting equipment for masonry, both available at tool-rental stores.

Surfaces to be painted for the first time must be scrupulously clean. Remove grease and oil from wood with mineral spirits. Seal knots, which may ooze sap, with a commercially available liquid compound, generally a blend of polyvinyl-butyral and a phenolic resin.

Metal surfaces are best cleaned with acetone or lacquer thinner, or with more expensive commercial degreasing com pounds. Less costly alkaline cleaning agents—strong detergents and trisodium phosphate—are also effective; however, these cleansers should be dissolved in hot water—150 to 200° F.

Steel and iron must also be stripped of any rust. Treat light rust with phosphoric acid in gel or solution form, available at hardware and paint stores. Not only do they clean away rust, they also etch the metal surface to increase paint adhesion. For serious rust, chip or scrape away the loose scales with a wire-brush attachment on an electric drill or with a wire brush and a hand-held scraper.

Bare concrete and masonry-block surfaces present two special problems. Porous types are difficult to clean—dirt and grime settle deep into crevices and can cause paint to blister or peel. Smooth, glazed masonry, on the other hand, pro vides a difficult surface for any new coating to penetrate.

To clean masonry, a stiff brush and strong detergent are effective, but a high- pressure spray-cleaning device—obtain able at tool-rental agencies—will save considerable time; it can pump a jet of water at 600 pounds per square inch to blast grime off a walkway or a wall. Once clean, bare masonry should be treated with muriatic acid to etch the smooth glazed surfaces—painters call it “raising a tooth”—for the new coating to grip. Muriatic acid also will dissolve efflorescence, a powdery alkaline substance that sometimes crystallizes on the surface of damp brick or cinder block. Afterward, rinse the surface with water to remove the acid. All masonry should be allowed to weather for as long as possible before painting; freshly poured concrete needs 60 to 90 days to release its alkalinity.

Plastic surfaces, though smooth, usually don’t need to be roughened for good adhesion. Static electricity, however, can sometimes cause paint to crawl, or alligator, on plastic surfaces—especially acrylic sheets—and must be removed with an alcohol wash (opposite, bottom). If the object being painted is portable, an electrically grounded metal screen placed underneath the plastic will help to keep it static-free during painting.

Surface preparation can be tedious and messy. When working with caustic cleaning agents, wear a hat, rubber gloves and old clothes. Wear a dust mask for fine- sanding projects, and don goggles and a charcoal-cartridge respirator before using muriatic acid indoors. Ventilate your work area well; also, before you start, clear away or cover anything that could be damaged by spills.

CHART OF: Making a Chemical Match with Paint over Paint

--Recoating an old surface. -- To determine whether a new paint can safely be applied over an existing paint, read down the first column to find the current surface paint. Below each possible new coating listed across the top, a solid dot indicates compatibility. An open dot indicates that the pairing is not recommended. If you don’t know what kind of coating s currently on the surface, experiment with a small amount of the new coating in an inconspicuous place.

Existing surface

Alkyd

Bituminous (roof coating)

Cement paint

Epoxy

Latex

Oil

Phenolic

Polyurethane

Silicone

Synthetic rubber

Vinyl

118

An Acid Scrub for Stains

119--

--Cleaning masonry. Use a wire brush to re move loose efflorescence. Then, wearing rubber gloves and protective clothing, paint on a solution atone part muriatic acid to nine parts water. To avoid dangerous foaming and splattering, always pour the acid into the water, never the water into the acid. Apply the solution to the masonry with a natural-bristled scrub brush, using plastic dropcloths, if necessary, to keep the solution off anything except the masonry. When the acid has stopped bubbling, rinse the surface thoroughly with water.

Draining Static Electricity:

--Ridding sheet plastic of static electricity. Loop one end of a bare copper wire through a piece of metal screening and attach the other end to an electric ground, such as a metal cold-water pipe or the small screw that secures a cover over an electric receptacle. Place the screen on the worktable, under the plastic sheet, and wipe the plastic with a mixture of 1 part isopropyl alcohol to 9 parts water. Although any soft cloth can be used to wipe on the alcohol, a well-wrung chamois will result in the least spotting.