Painting Wood Siding Tips and PreparationUpdated: Feb. 27, 2023
Prep tips and techniques for wood siding and trim
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Overview: Good prep means less painting
Filling gaps and holes
Proper prep before painting will make the job last longer. It saves you time and money in the long run.
Tired of your paint job peeling when it’s only two years old? Paying close attention to the prep work can add years to the life of your finish coat of paint. In this article, we’ll give you tips and techniques on cleaning, scraping, filling and priming before painting wood siding. We’ll also help you identify and solve some specific problems that may have caused your house paint to fail prematurely.
Applying the paint is easy. But creating a sound, dry surface for the new coat of paint is tough and time consuming. However, it’s the key to any successful exterior paint job.
Painting Wood Siding Tip 1: Pressure washers clean deep and fast
Painting Wood Siding Photo 1: Pressure wash to remove dirt
Pressure washing removes loose paint and built-up grime and improves paint adhesion. Use the high pressure carefully, especially around windows. High-pressure water can break the glass. Avoid directing water up under the laps, and keep the nozzle at least 16 in. away from the wood.
Paint just won’t stick to dirty or dusty surfaces. You’ll need to clean it even if there’s very little scraping to do, and the fastest way is with a pressure washer. You can rent a pressure washer (Photo 1) from a rental store and get a lot of loose paint and grime off your old painted surfaces fast.
These washers kick out a hard stream of water, so try it out on an inconspicuous spot on the house to get the hang of handling the wand. When pressure washing a house, be careful not to hit windows (they can break), and don’t work the spray upward under the laps of siding. Remember, this is for cleaning, not blasting all the old paint off. Of course, some of the old loose paint will fall off, but too much pressure will gouge the wood.
Don’t try to pressure-wash while standing on a ladder. The recoil can knock you off balance. And finally, keep in mind that you won’t be able to do any scraping and sanding for a couple of days until the surface dries thoroughly.
If the prospect of using a pressure washer is just too intimidating, you can get a stiff brush on a pole and a bucket with mild detergent and scrub the surfaces. Follow the scrub immediately with a rinse from your garden hose.
Lead Paint Caution: Houses built before 1978 may contain lead paint. Before disturbing any surface, get a lab analysis of paint chips from it. Contact your local public health department for information on how to collect samples and where to send them.
Painting Wood Siding Tip 2: Keep your scraper sharp
Photo 1: Scrape with the grain
Follow the grain of the wood with long strokes. If nails are sticking up, pound them in. If the nail won’t stay put, pull it and drive a new galvanized nail 1/2 in. away.
Photo 2: Keep your scraper sharp
Pull down firmly with a sharp scraper to remove loose paint. Remember, you don’t need to remove all the paint, just the stuff that flakes away with the scraper. Keep your scrapers sharp with a fine file. Tip: Don’t scrape wet wood. You’ll tear away the fibers and dig deep gouges into the wood.
Photo 3: Use small scrapers too
Get into tight areas with a small 1-in. scraper. Heavy buildup of paint in corners will eventually crack and let moisture in. Scrape and cut excess paint out of the corners with a sharp putty knife.
Old, flaking paint must be scraped from your wood surface or your new paint will eventually let go. Make sure the surfaces are dry. Then scrape in the direction of the grain to avoid tearing the wood fibers and creating an unstable surface for your primer. A sharp scraper is the best. You can buy hardened steel scrapers (Photo 1), or for about twice the price, you can buy carbide scrapers (Photo 2).Good-quality scrapers all have replaceable blades. You can easily sharpen a steel scraper blade using a fine metal file. The carbide blades last up to 10 times longer but must be sharpened with special tools. Buy replacement blades to have on hand.
Painting Wood Siding Tip 3: Fill large gouges before priming
Photo 1: Fill holes
Overfill each wood siding repair and then shape it, once it has set, with a file, sharp chisel and sandpaper. Blemishes deeper than 1/2 in. will need additional applications. Once the patch is dry, shaped and sanded, prime it to protect it from moisture.
Before painting wood siding, fill large holes and gouges with a two-part resin filler such as Minwax High Performance Wood Filler. You have to mix these setting types of fillers, but they stick better to wood than other fillers. Remove any paint around the area before filling. You can fill nail holes with them too.
For small, shallow blemishes, use an exterior spackling compound. Cracks can be filled with exterior caulk after priming.
Painting Wood Siding Tip 4: Sand the ridges
Photo 1: Smooth paint edges
Feather the edge of your scraped areas with a power sander to get rid of sharp edges. These ridges can break the finished paint surface later and allow moisture to get behind the paint. After sanding (use 80 to 100 grit), use a dry brush to whisk away any surface dust, especially on horizontal surfaces like windowsills.
Power sanders cut fast. Use a 60-grit paper for heavy ridges followed by 100-grit for a smooth look. Also sand shiny, old paint surfaces to give the topcoat better bite. Tip: Sand all old, bare wood. Paint won’t stick to wood once it has weathered.
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Painting Wood Siding Tip 5: Use a stain-blocking primer
Photo 1: Spot prime first
Spot-prime nailheads and knots with a special stain-blocking primer to prevent unsightly bleed-through from rust or wood resin. A pigmented shellac (BIN, for example) is a good product for this use.
Photo 2: Prime all bare wood
Work the primer into cracks and especially where trim pieces meet siding. Even out the primer with long brush strokes and check for drips. Remember to work the primer under the siding laps and into tight spaces and hard-to-see spots. Surfaces that still have old paint that’s adhering well don’t need a primer.
You can choose either an oil or a latex primer and get great results. Oil primers, however, are generally more effective on new wood, metal and previously chalked surfaces. If you’re priming over bare woods that have a high tannin content, such as cedar and redwood, ask your paint supplier for a special stain-blocking exterior primer.
Stain-blocking primers will prevent a “bleed-through” of tannin through the primer and the topcoat and stop old, rusty nailheads from bleeding through as well (Photo 1).
Always try to avoid priming or painting wood siding in direct sun. The extra heat can dry the primer and paint too quickly and prevent adequate penetration. It can also cause oil paints to develop blisters that’ll ruin the skin of the finish coat of paint. Tip: Your paint supplier can add pigment to your primer to get it close to the topcoat color. This is especially helpful when your topcoat is a darker color. You may be able to cover the primed area and old paint surface with one coat of paint.
Painting Wood Siding Tip 6: Caulk cracks to keep out moisture
With all the caulks available at home centers and paint supply stores, it can be hard to know what kind of caulk to buy to seal gaps. Most professionals agree that acrylic caulk and siliconized acrylic caulk are the best for caulking around windows and doors and against corner boards. These caulks are paintable, long lasting and easy to clean up.
Caulk after priming. The caulk sticks best to a primed surface. Squeeze enough caulk into the gaps to get a smooth bead that fills the void. Excess caulk will look sloppy and will only increase the possibility of working loose over time.
Diagnose Common Paint Problems:
Poor Prep and Excess Moisture
Take a good look at your house before you start painting. You may have to correct some of the wrongs that the previous painter overlooked in desiring to get the job done fast. You may have peeling paint as shown in Figures A – C.
Poorly prepared surface
You may notice the finished coat of paint peeling away from another coat underneath (Figure A). This is the result of painting over a poorly prepped surface. Usually the surface wasn’t cleaned before painting or the primer coat was left too long before the finish coat of paint was applied. You can remedy this situation by firmly scraping away the paint to get at the surface below. Scrape until you get down to a solid surface that may be part bare wood or a sound, previously painted surface. Sand and clean the surface thoroughly, let it dry, and prime all bare wood and spot-prime any small bare spots.
Sometimes you find paint and primer falling away from the bare wood surface (Figure B). This is most likely caused by water getting behind the wood or even moisture from the home’s interior. You can’t repaint here unless you stop the source of the moisture migrating through the walls, especially from bathrooms and kitchens. Adding ventilation such as an exhaust fan in the room can frequently help the problem. Many older homes have no vapor barrier, but sometimes applying a vapor barrier paint on the inside will do the trick. If you’re stymied, consult a building contractor to help you solve the problem. Then scrape and sand to remove the loose paint.
Another common problem is cross-grain cracking, sometimes referred to as “alligatoring.” The source of this problem is usually paint buildup from several layers of oil-based paints. Unfortunately, there’s no magic bullet for this fix. You’ll have to scrape off all the old paint down to bare wood and then sand the surface. Take precautions and gather this paint onto drop cloths or plastic for disposal because it may contain lead. Don”t keep applying layers of paint to areas that don’t need it such as porch ceilings and eaves where the surfaces are protected. Often a good cleaning is all they need.
You may find dark mildew spots on the finished coat of paint along shady areas of the house. Buy a special cleaner at paint supply stores to scrub the surface. You can usually stop mildew by increasing exterior airflow in that area, trimming plants close to the house, and channeling water away with gutters and downspouts. If the area is tough to air out, you can get mildewcide additives for your finish coat of paint.
Another problem is chalking, where old paint surfaces get powdery. This is natural for old paints, but you’ll have to power wash or scrub the surface and do some light sanding to remove the chalking before applying paint.
Required Tools for this Project
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
- Caulk gun
- Orbital sander
- Paint scraper
- Putty knife