How do you prepare paint surfaces?

There are several important preparation steps for both interior and exterior paint jobs. This list does not cover possible circumstances encountered on every painting project—only the more common ones.

Surface preparation

• Wash the walls if accumulated dirt is a problem. Any household detergent will work. Use TriSodium Phosphate (TSP) for grease or heavy cigarette smoke buildup. If you use powdered TSP, rinse it off with water. Use chlorine-based bleach on mildew.
• Use a scraper or a putty knife to remove all loose paint and sand down the rough ridges where the paint has broken off. Dig out any cracks a fraction of an inch on both sides to form a ‘V’ shape. This increases the contact area for your patching material to adhere to.
• Apply filler to cracks, holes, and heavy paint ridges. Two thin patch coats are always better than one thick coat. Spackle that’s too thick tends to shrink and crack as it dries. I like to use Fix-All for the first patch coat in deep holes and cracks. Fix-All is difficult to sand, so keep it in the crack or hole, slightly recessed below the wall’s surface, then follow it up with a thin coat of spackle.
• If the wall has a texture other than smooth, your patch should match that texture as closely as possible. You can create a stippled effect by dabbing the surface with a sponge or stiff-bristle brush while the patching material is still tacky. You can also roll a coat of stipple paint or thinned-out joint compound over the dried-out patch.
• Use latex caulk to seal joints around door jambs, window casings, and baseboards. Broad and deep joints may require more than one application. Use a damp rag to wipe off any excess caulk.
• Sand down all glossy surfaces or treat them with liquid sandpaper. This provides a roughened surface or tooth for good adhesion of your subsequent coat.

Primer application

No matter what type of surface you’re painting, it must be clean, free of loose or cracked paint, rust, scale, oil, grease, dirt, mildew, and chemical residue before primer application.

Primer or undercoat has two primary purposes:

1. To seal the substrate, preventing its chemistry from migrating into and interfering with the chemistry of the finish coat.
2. To help bind the finish coat to the surface being painted.

There is a specialty primer for just about every type of surface—wood, masonry, metal, and plastic. The primer serves as a foundation that supports adhesion of the finish coat. Understanding this should help you understand the importance of primer. The best primer available will be your best choice for any paint job.

House paint preparation

Before any other work begins, check for leaks and moisture entering the substrate. Repair any roof leaks, gutters, windows, and leaky plumbing. Damp basements and other excessively humid interior rooms are other familiar sources of moisture in the substrate.Use vents and dehumidifiers as necessary. Unless the source of moisture in the substrate is found and eliminated, the quality of your paint job will be compromised.

• All exterior surfaces will need to be washed to remove mildew, dirt, and excessive chalking. Consider using a power washer if more than 500 square feet of surface needs to be cleaned. Mildew has a blotchy and powdery appearance. It is a common living organism found in damp areas that receive little sunlight. Any surface with mildew must be thoroughly sterilized before painting. Wash the mildewed surface with a 1-quart household bleach solution in a gallon of water.
• Chalking is a loosely bound powder that forms on the paint’s surface. Chalking occurs when sunlight and moisture break down the paint binder. Scrape, sand, and wash off all chalking before primer application.

• On stucco, thoroughly scrape off loose paint, and repair all cracks and holes. Fill gaps around windows, door casings, or where two materials meet, such as at the foundation line or where wood meets masonry. Use caulk or elastomer-based patches to fill these cracks. Cracks of 1/16” wide or more extensive should be chiseled out a fraction of an inch on both sides to form a ‘V.’
• This increases the contact area for the patching material you will be using. If you use a non-textured stucco patching material, texturize the patched area to match the existing stucco. You can use a coarse fiber brush, like a scrub brush, for this purpose.
• Rub the brush over the partially set patch in a circular motion until the repair looks like the rest of the wall. If you are using an elastomer-based patch, use an old brush to dip it in water and feather the patch’s edges. Remove any efflorescence (calcium deposits) with a stiff-bristle brush. Then neutralize the salt with a 5% muriatic acid solution. Rinse thoroughly with clear water.
• Use a good exterior primer or surface conditioner for previously painted stucco. Because of the chemical reaction in the newly applied plaster, the new stucco must be allowed to cure properly before being primed with an alkali-resistant primer.
• On wood surfaces, thoroughly scrape off all loose and cracking paint. Sand it to remove any remaining loose paint and create a smoother surface. Consider stripping if more than 25% of the coating is cracked or peeling. In stripping, chemicals or heat are used to remove all existing layers down to the substrate.
• When multiple coatings are present, stripping may be the best way to prepare the surface. However, because of the brutality of the stripping process, a cost-benefit analysis will need to be conducted.
• Inject caulking compound into cracks around windows, doors, and open vertical seams. Use a good-quality exterior wood primer. Do not caulk the horizontal seams between siding planks. Because siding planks overlap, this opening is rarely a source of leaks.
• On the contrary, caulking these joints can trap moisture in the substrate. 
• Never use a rigid patching material, such as spackle, in a shallow depression on an exterior surface. Exterior surfaces (mainly wood) undergo significant contraction and expansion due to fluctuations in substrate humidity and temperature. Spackle dries hard and will become brittle. It will crack and cause premature paint failure.
• If you must use patching to smooth out surface imperfections, like paint ridges, use elastomer-based products that move with the substrate as it expands and contracts.

• Galvanized metal comes from the factory with a residue of the manufacturing process that prevents good paint adhesion. It is best to let galvanized metal surfaces weather for about six months. Weathering neutralizes the surface, making it more ready to accept paint. If you have to paint galvanized metal right away, etch the surface with a mild acid such as vinegar. Rinse the surface thoroughly and then apply a coat of galvanized metal primer.
• When painting iron or steel, the most important thing is good contact between the surface and the coating. These metals rust when air and moisture get under the protective layer. Rust is like cancer. Once it is in the metal, it is virtually impossible to remove. That’s why surface preparation is so necessary. Use a good rust-inhibitive primer.
• Rusting metal must be sanded to remove loose rust scale, then cleaned and primed with rust-inhibitive primer. Punch in rusting nails to set them slightly below the surface. Spot prime the nail heads with a rust-inhibitive primer, then fill the nail holes with epoxy filler.

In my past thirty years as a painting contractor, I have been called on to help with many prematurely failing paint jobs. Amateurs handled some of these jobs, and professionals dealt with others, but all had one thing in common: one or more skipped or improperly performed surface preparation steps.

Good surface preparation is essential for a long-lasting paint job. The best paint, good painting technique, and a delicate brush will not compensate for a single needed but skipped preparation step. Skimp on surface preparation, and you will cut years off the useful life of your paint job.