How to Prep Exterior Wood Siding for Paint
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Short answer: wash the siding and let it dry for 2 days, scrape everything loose, sand the edges flat and any glossy areas dull, fix rot and reset nails, caulk the joints, then spot-prime every patch of bare wood. That's the order, and none of the steps are optional - when exterior paint peels early, the cause is almost always a prep step that got skipped, not the paint.
Pros put 70-80% of an exterior job's hours into prep. Here's each step, why it exists, and where corners actually get cut.
The sequence
Step by step
1. Wash. Dirt, chalk, and mildew are bond-breakers - paint over them and it sticks to the grime, not the wood. A garden-pressure rinse with a siding cleaner (or TSP substitute) and a soft brush is enough; if you use a pressure washer, keep it under ~1,200 PSI and a wide fan angle. High pressure drives water deep into the boards and furs up the grain - it's a paint-stripping tool, not a washing tool.
2. Dry - a full 48 hours of decent weather. Paint and primer over damp boards is the classic slow-motion failure: the moisture eventually pushes the film off from behind. Wood should read below ~15% on a moisture meter if you have one.
3. Scrape. Everything that lifts to a carbide scraper was going to lift under new paint anyway. Chase every bubble and edge until the scraper stops finding loose material. (Pre-1978 house? Assume lead paint - no dry power-sanding; follow EPA RRP practices or hire it out.)
4. Sand. Two different jobs: feather the scraped edges flat with 60-80 grit so they don't telegraph through, and dull any sound-but-glossy paint with 100-120 so the new coat can grip. Full guidance by grit is in what grit sandpaper to remove paint. For a whole peeling wall, a purpose-built stripping tool beats sheet sandpaper by an afternoon - the Wagner PaintEater exists for exactly this, and our exterior paint-removal sander guide sorts the options by wall condition.
5. Repair and caulk. Dig out soft rot and fill with exterior epoxy or two-part wood filler (replace boards where rot is structural); set proud nails and rust-spot them; then caulk vertical joints, window and door casings, and trim intersections with paintable acrylic-urethane caulk. Don't caulk the horizontal underlaps of clapboard - those gaps are the wall's drainage and ventilation.
6. Spot-prime. Every area of bare wood, filler, and caulk gets primer - topcoat alone over bare wood flashes dull and fails early. Cedar and redwood want a stain-blocking primer (tannin bleed turns white paint tea-brown); glossy or chalky-but-sound surfaces bond best with an adhesion primer like KILZ Adhesion. If more than half the wall ends up bare, prime the whole thing instead of spotting.
Then paint - two topcoats, and the whole exercise is only as good as what goes on top: see best exterior paints for wood siding for the paint-and-primer picks, or best outdoor house paints for whole-exterior systems.
The corners people cut (and the bill later)
| Skipped step | What happens | When |
|---|---|---|
| No wash | Paint bonds to chalk/mildew film, sheets off; mildew grows through | 1-2 years |
| Painted damp wood | Blisters, then peeling from behind | First summer |
| No scuff on glossy paint | New coat scratches or peels off in ribbons | Months |
| Bare wood, no primer | Flat blotches now, early failure + tannin stains later | Immediately visible |
| Caulked the drainage gaps | Trapped moisture, rot behind the siding | Years - expensive |
FAQ
Do I need to sand wood siding before painting? You need to sand two kinds of places: feathering where old paint was scraped (60-80 grit) and de-glossing sound shiny paint (100-120). Sound, matte, clean paint doesn't need sanding - new acrylic bonds to it fine after washing.
Can I just pressure wash instead of scraping? No - a pressure washer strong enough to remove failing paint is strong enough to bruise the wood and inject water into it. Wash gently to clean; let the scraper and sander do the removal, dry.
How soon after washing siding can you paint? Give it 48 hours of dry weather minimum - longer after heavy rain or if the washer got aggressive. The failure you're avoiding (moisture trapped under fresh paint) doesn't show up until the sun cooks the wall.
Do I have to prime old wood siding before painting? Prime everywhere the substrate isn't sound existing paint: bare wood, filler, caulk, chalky areas, and all cedar/redwood (stain-blocking). "Paint-and-primer-in-one" products handle sound painted surfaces, but they are not a substitute for real primer on bare exterior wood.
