Post-Build Floor Prep & Paint

10x12 Gable Shed — 222 Gibson St SE · Phase 5, before move-in · Getting the PT deck ply from as-built to painted.

Order of operations

Confirm dry — sprinkle test
Test paint adhesion on an offcut
Set proud nail heads flush
Fill nail holes, tearout and voids — exterior wood filler
Ramp the 1/8" seam step — feather on the low side only
Sand 80–100 grit, then vacuum thoroughly
Seal the seams — flexible sealant, full cure per label
Two coats porch & floor enamel

Sealant goes on before paint, not after — you want the enamel over it, sealing its edges. Sanding comes before sealing so you're not cutting into cured sealant.

1 · Is it dry?

Sprinkle test. Flick water onto the surface in a few places.

What happens	Verdict
Drops bead up and sit	Too wet. Wait
Drops soak in within ~30 seconds	Dry enough

Installed August 2026 and painted before move-in, the floor gets months of drying — this should pass comfortably. Test anyway. Enamel over wet PT blisters and peels no matter how good the prep was.

2 · Nail heads — flush, never sunk

A head driven through the face veneer loses most of its shear value, and this floor is structural. The target is flush, not countersunk.

Tool	Use
Smooth-faced hammer	The answer. One or two light taps brings a proud head flush without marking
Milled (waffle) face framing hammer	The cause. That grid stamps into the ply — this is almost certainly what's marring the surface
Nail set, 1/8" tip	Strike the tool, not the wood. But it's built to drive heads below the surface — use it only for a final light tap

If you're chasing many proud heads, the gun's depth-of-drive is shallow or pressure is sagging. Set it on a scrap and re-check when the compressor cycles or the temperature changes. Light hammer marks sand out in step 6; a crater through the veneer does not.

3 · The 1/8" seam step — as-built

Condition: at one seam, one sheet sits a solid 1/8" below its neighbor. Cause not diagnosed before nail-off — most likely a joist installed with less crown, since 1/8" is well outside the ±1/32" mill tolerance for 23/32 ply. Sheets are fully nailed at 6"/12", so pulling and shimming was rejected: extracting ring shanks from PT ply does more damage than the step.

The ramp — feather the low side only

Exterior wood filler, or a patching compound rated for wood subfloors
Thin lifts. Two or three passes — 1/8" in one application cracks
Feather 8 to 12 inches back from the seam. That's a slope near 1:100, imperceptible underfoot and invisible under paint
Sand with a long block or orbital so you cut the ramp flat rather than following its contour
Prime and paint

4 · Filling the seams

The seams were cut roughly 1/16" scant, and the deck carries about 0.16" of racking that migrated into the cut pieces at the south and east. The gaps are real and worth closing.

Product	Movement	Paintable	Floor traffic	Verdict
Self-levelling polyurethane (Sikaflex SL, NP1 SL)	±25–35%	Yes	Rated for it	Best. Flows flat on its own — no tooling, no ridge under the enamel
MS polymer / hybrid (OSI Quad Max, Loctite PL)	±25–50%	Yes	Yes	Nearly as capable, far easier to tool and clean up
Siliconized acrylic latex caulk	±7.5%	Yes	No	Too soft — scuffs, collects dirt
Silicone	High	No	—	Disqualified: won't take paint

Three things decide whether it lasts

Timing. The PT went down wet and shrinks as it dries — gaps will be wider by spring than at install. Fill at Phase 5, once the movement is done. Filling early tears it loose.
Wider joints survive better. Counterintuitive but true: sealant needs width to stretch across. A 1/16" joint with 1/32" of seasonal movement works the sealant at 50%; a 1/8" joint works it at 25%.
Don't fill deep. Sealant bonded on both sides and the bottom can't stretch — it tears. The seams sit directly on a joist, so run a shallow bead and tool it flush rather than pumping the joint full.

Honest expectation: most joints hold indefinitely; one or two may show a hairline separation after a few seasons. Touch-up is a five-minute job.

5 · Sanding — this matters more than filling

PT plywood is rough, and the grain raises as it dries. A pass with 80 to 100 grit does more for the finished look than any seam treatment, and it materially improves paint adhesion.

Then vacuum thoroughly. Enamel over sanding dust is a bad finish, and PT dust is worth not breathing or leaving behind — wear a mask and clean up properly.

6 · Paint

Item	Spec
Product	Porch & floor enamel, 1 gal + roller kit
Coats	Two
When	End of Phase 5, before move-in
Cost / order	~$90 · Order 3 (line `o3-16`)
Coverage needed	120 sq ft × 2 coats

Prime first if the adhesion test suggests it — the scrap test in step 1 tells you. Let the sealant cure fully before the first coat; polyurethane in particular needs its full stated time, not just skin-over.

Provenance

Written Aug 19, 2026 from the floor-prep discussions during deck installation. As-built conditions recorded here — the 1/8" seam step, the ~0.16" of deck racking, the scant-cut seams — are specific to this build and were logged as they happened. Related: Base & Deck Construction for the deck itself, Build Guide Phase 5, Orders & Deliveries for the enamel line.