How to Paint Exterior Wood: Prep, Primer, and What Fails First
How to paint exterior wood siding right — substrate prep, stain-blocking primer, back-priming, and the failures that show up in two years if you skip steps.
Exterior wood doesn’t fail because of the paint. It fails because of the prep. Skip the wash, skip the scrape, skip the stain-block on a knot, and you’ll be back on the same ladder in three summers wondering what went wrong.
TL;DR
- Wash: soft-wash with deck cleaner, then 48–72 hours of dry weather
- Repair: scrape every flake, sand chalky paint, feather the edges flat
- Spot-prime: stain-blocker on every knot, every bare patch, every cut end
- Full prime: exterior bonding primer over the whole wall
- Caulk: stretchy elastomeric caulk in every seam (not standard acrylic)
- Paint: 100% acrylic exterior, two coats, brush-and-back-roll
- Cure: 30 days before pressure-washing the painted wall
- Skill: hard. Wrong primer choice and skipped back-priming both kill the job
Wood types and what each one demands
Cedar lap siding. Default West Coast siding. Heavy tannin, especially red cedar. Stain-blocking primer (Cover Stain or BIN) on every bare patch. One spot primed with generic bonding primer telegraphs amber rings through two white coats inside a month.
Redwood. Even more tannin-heavy than cedar. Two thin coats of Cover Stain on bare areas, sanded between.
Knotty pine. Pine isn’t tannin-heavy, but the knots are. BIN or Cover Stain on every knot. Skip one and you’ll see a yellow halo by spring.
Pine T1-11 panels. Plywood-faced grooved siding. Grooves catch water; the face checks under UV. Two-coat primer, lots of caulk, back-prime cut ends.
Smooth vs rough-sawn. Smooth paints faster but shows brush marks. Rough-sawn hides imperfection and eats 25–40% more paint. 3/4-inch nap on rough, 1/2-inch on smooth.
Pre-primed siding from the yard. Easy mode. Spot-prime field cuts, caulk, finish coat.
Old painted siding. Hardest case. Could be 1990s latex over 1970s alkyd over 1950s lead, three incompatible films stacked. Run the tests below.
If your siding rings dull and weighs a ton, it’s fiber-cement (Hardie, Allura), not wood. Different primer, no tannin, no back-priming. This guide is for the wood case.
Diagnose the existing finish first
Three tests. Five minutes. Saves a wasted project.
Chalk test. Wipe a dark cloth across a dry painted board. White powder on the cloth means UV has oxidized the binder. Heavy chalk means the existing film won’t hold a topcoat without bonding primer (Peel Stop). Latex over chalky alkyd lifts the chalk and peels with it inside two summers.
Peel test. Score a small X with a utility knife. Press painter’s tape over it, smooth, yank fast. Clean tape, the film’s bonded. Paint comes with the tape, you’ve got scraping ahead.
Mildew test. Daub 4:1 water-to-bleach on any black or grey-green discoloration. Disappears in 60 seconds, it’s mildew and has to be killed before paint. Doesn’t budge, it’s dirt.
Materials and tools
- 100% acrylic exterior paint (Aura Exterior, Emerald Exterior, or Marquee Exterior)
- Zinsser Cover Stain for spot-priming; Zinsser BIN for tannin-heavy spots and knots
- Zinsser Peel Stop for chalky surfaces
- Stretchy elastomeric caulk (Sashco Big Stretch, OSI Quad Max, or DAP Dynaflex Ultra). Never standard acrylic.
- Bondo or Minwax exterior wood filler
- TSP or deck cleaner; lead test kit ($15) for pre-1978 homes
- Pressure washer (1,500 PSI ceiling on wood), 25-degree tip
- 2-inch and 4-inch scrapers, 80- and 120-grit sanding blocks, random-orbit sander
- 2-inch and 2.5-inch angled sash brushes (Wooster Silver Tip or Purdy Clearcut)
- 1/2-inch nap roller for smooth, 3/4-inch for rough-sawn
- Airless sprayer (515 or 517 tip) if you have one; caulk gun; extension ladders
- N95 minimum, P100 for pre-1978 paint, eye protection always
Step 1: Wash the wall
Soft wash with a deck cleaner, rinse, then 48–72 hours of dry weather before anything else touches the wood.
Cover plants. Mask windows. Mix deck cleaner per label (Krud Kutter Deck and Fence Cleaner, or TSP at 1/2 cup per gallon). For mildew, brush on 3:1 water-to-bleach with a splash of dish soap, dwell 15 minutes, rinse.
Pressure wash at 1,500 PSI max, 25-degree tip, 18 inches off the wall. Higher pressure fuzzes the grain and you’ll fight that fuzz forever. Work top down. Angle across seam joints, never into them.
Then 48–72 hours of warm dry weather. Wood that looks dry on the surface is still wet inside the boards. Primer over wet wood blisters off in three weeks.
Step 2: Scrape, sand, and feather the edges
Scrape every loose flake, sand the chalky paint until the edges feather flat. No bright lines under the next coat.
Walk the wall with a 2-inch scraper. Anything loose comes off. Don’t gouge. Sand the bare patches with 80-grit until they feather flat into the surrounding paint. A bare-wood ring with sharp edges telegraphs through the topcoat as a visible rectangle. Sand the rest with 120 to break the gloss. Vacuum or tack-cloth the dust.
Cracks and splits get Bondo wood filler (sands flat at 30 minutes). Set nail heads, spot-prime, fill. Rotted wood you cut out and replace.
Pre-1978 home? Stop and read the lead section below before you scrape.
Step 3: Spot-prime knots and bare wood
Zinsser Cover Stain or BIN on every knot, every bare patch, every cut end. Stain blocker before full primer.
This is the step that separates a five-year paint job from a fifteen-year one. Every bare patch, every knot, every cut end gets stain-blocking primer before full primer.
Cover Stain is the workhorse: oil-based, blocks tannin, recoats in two hours. BIN is shellac, blocks anything, dries in 45 minutes, but it’s brittle on exterior so keep it to spot work. On heavy tannin (redwood, knotty cedar) lay two thin coats of Cover Stain on the worst spots, sanded between.
Brush it on with a 2-inch sash. Don’t worry about brush marks. Full primer covers them.
Step 4: Caulk the seams
Sashco Big Stretch in every seam. Then a full coat of exterior bonding primer over the whole wall.
Caulk after spot-priming, before full primer. It bonds better to a primed surface, and the spot-primer seals any tannin in the joint.
Run a bead in every seam where two boards meet: corner trim joints, window and door casings, visible lap interlocks, T1-11 grooves at the ends. Sashco Big Stretch is my default. Moves 500% without splitting. OSI Quad Max and DAP Dynaflex Ultra are also rated for siding. Standard acrylic caulk is rated for 25% movement and fails inside three years. Don’t substitute.
Tool the bead flat with a wet finger. Wipe excess while it’s wet; caulk smears clean before it sets and tears the next day.
Step 5: Full prime the whole wall
A full coat of exterior bonding primer goes over the entire wall: spot-primed patches, caulk, and old paint. Unifies the surface.
Peel Stop is the spec over chalky alkyd. Sherwin-Williams Exterior Latex Wood Primer or Benjamin Moore Fresh Start Exterior cover sound previously painted wood. The can has to say “exterior” and “wood.” Interior primer breaks down under UV inside a season.
Brush the cut-in with a 2.5-inch angled sash. Roll the field. Or spray and back-brush with two people: one running production, one back-brushing while the primer is wet. Spray-only leaves a film sitting on top of the grain instead of in it.
One coat is enough on a sound wall. Heavy chalk or high tannin may want two thin coats. Cure 4 hours touch-dry, 24 hours recoat.
Back-priming. Most-skipped step in exterior wood work. Water gets behind siding through every cut end, freezes inside the board, splits it, and pushes the topcoat off from underneath. Installing or replacing siding, prime all six faces before it goes up. On an existing wall, prime every cut end you create.
Step 6: Two finish coats
100% acrylic exterior. Aura, Emerald, or Marquee. All three carry enough binder to handle UV and seasonal movement on residential siding. Cheap contractor-grade chalks fast on a south face.
Brush the cut-in first, roll behind it, keep a wet edge. Don’t stop mid-wall. Start at one end of a board run and finish to the other. Stopping mid-wall is how you get lap marks the second the morning sun hits them.
Watch the dew point. Most acrylics want 50°F minimum, 90°F maximum, and at least 5°F above dew point so the film dries before condensation hits. South-facing walls get painted early morning or late afternoon, never noon. Wood in direct sun runs 30°F hotter than the air and acrylic flashes the solvent before the film levels.
Sixteen hours between coats. Some cans say four. Don’t believe them on exterior.
The second coat fills holidays and builds the mil thickness the manufacturer specs (4 mils wet, 1.5 dry, per coat). Look at the wall in raking morning sun before you call it done. Touch up with a brush from the same can.
Step 7: Cure
Touch-dry in 2–4 hours. Recoat-dry in 16. Full cure runs 30 days. During cure, don’t pressure-wash, don’t lean ladders against the wall, don’t run sprinklers onto it. A soft film picks up dirt and damage that locks in permanently.
Common mistakes
- Skipping stain-blocker on knots and bare patches. Yellow halos by next spring.
- Latex over chalky alkyd without bonding primer. Topcoat peels with the chalk inside two years. Run the chalk test; if positive, Peel Stop before topcoat.
- Spray-only on rough siding. Paint sits on top of the texture instead of in it. Three years to failure. Back-brush or back-roll.
- Standard acrylic caulk in siding seams. Fails at 25% movement; wood siding moves more.
- Painting late in the day in cool weather. Surfactant leaching. Four hours of warm dry air ahead of sundown.
- Skipping back-priming on cut ends. Water wicks into end-grain, freezes, splits the board.
- Painting weathered grey wood without sanding to fresh grain. UV-damaged fibers are dead and they let go of any paint laid over them.
- One thick coat instead of two thin. Thick coats sag, alligator, never reach proper cure.
The lead question on pre-1978 homes
Pre-1978 paint contains lead until proven otherwise. Test scraped chips from a hidden area with a $15 hardware-store kit first.
If positive, EPA Lead-Safe rules apply: no dry sanding, no open-flame strippers, no power-washing chips into the yard. Wet-scraping with chips caught on contained drop cloths. HEPA vacuum cleanup. P100 respirator, disposable coveralls, wash up before going inside.
A homeowner can legally do their own house without certification. A hired contractor cannot; they must be EPA Lead-Safe Renovator certified. If a painter on a pre-1978 home says “we don’t worry about that,” walk away.
Maintenance and longevity
Properly painted exterior wood lasts 8–12 years on north or east faces, 6–8 on south. Early failure signs: chalking south, mildew north, peeling at the bottom three feet (ground moisture), splits at joinery seams (caulk failure).
Wash annually with a soap injector and a soft pole brush. No pressure washing in the first year of cure; after year one, 1,000 PSI on a 40-degree tip is the ceiling. Re-caulk seam failures the season they show. South-face refresh at year 6 if it’s chalking ahead of the rest.
Will it bite you in two years? Only if you skipped the wash, painted over chalk without a bonding primer, missed a knot with stain-blocker, or used the wrong caulk. Get those four right and the wall holds for a decade.