Most people emphasize the "look" of paint finishes. After all, this is what they see when the paint has dried. But for a long-lasting and dazzling appearance to your freshly painted home, be sure you understand the importance of proper surface preparation.
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.
Wash the walls if accumulated dirt is a problem. Any household detergent will work. Use TriSodium Phosphate (TSP) for grease or a heavy build-up of cigarette smoke. If you use powdered TSP, be sure to 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 something other than a smooth texture, 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 on top of 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.
No matter what type of surface you’re painting, it has to 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:
to seal the substrate to prevent the chemistry of the substrate from migrating into and interfering with the chemistry of the finish coat.
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, 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.
Exterior Paint Preparation
Before any other work is begun, check for leaks and any moisture getting into 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 to clean. Mildew has a blotchy and powdery appearance. It is a living organism common in damp areas that get little sunlight. Any surface that has mildew must be sterilized entirely before painting. Wash the mildewed surface with one quart of household bleach mixture in a gallon of water.
Chalking is a loosely bound powder that forms on the paint’s surface. Chalking happens when the paint binder is destroyed by sun and moisture. Scrape, sand, and wash off all chalking before primer application.
On stucco, thoroughly scrape off loose paint repair all cracks and holes. Fill gaps around windows, door casings, or where two materials meet, such as at the foundation line or 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 make it approximate 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, take an old brush, dip it in water, and feather out the patch’s edges. Remove any efflorescence (calcium deposits) with a stiff-bristled brush. Then neutralize the salt with a 5% solution of muriatic acid. Rinse thoroughly with clear water.
Use 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 correctly and then primed with 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 brutal nature of the stripping process, a cost vs. benefit analysis will need to be evaluated.
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 rarely is a leak source.
On the contrary, caulking of these joints can cause moisture to be trapped 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 a significant contraction and expansion due to humidity and heat fluctuation in a substrate. 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 get rid of. 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 rust inhibitive primer and 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. Some of these jobs were done by amateurs and some by professionals, but all had one thing in common— one or more skipped or improperly done 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.