Primer Overspray on Subfloor
Washing dried wall-primer overspray off a subfloor before self-leveling so the leveler's primer bonds to wood, not film.
Seal Subfloor Holes With Expanding Foam Before Pouring Self-Leveler
Self-leveling underlayment finds every hole in the subfloor and pours itself into the room below. Seal the big gaps with expanding foam first — it mushrooms tight against the plywood — then trim the excess flush the next day with an oscillating tool.
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The Problem
Self-leveling underlayment is basically water that sets — that's the point, and it's also the hazard. It runs to and through every plumbing penetration, gap, and knothole in the subfloor, and the first sign of a missed hole is leveler raining through the ceiling of the room below.
The Technique
- Before mixing any leveler, hunt down every hole and gap in the subfloor — plumbing penetrations, seams, damaged spots.
- Fill the large ones with expanding spray foam, using the big-gap formulation: it expands a lot, which is exactly what you want, mushrooming into the void and sealing tight against the plywood.
- Let it cure, pour your self-leveler with confidence — nothing's escaping into the room below.
- The next day, slice the excess cured foam flush with an oscillating multi-tool.
Pro tip: Foam beats tape-and-caulk for this job because it seals blind — you don't need to reach or even see the underside of the gap; the expansion finds the shape of the hole for you. Do the whole perimeter check in one pass before the pour, because there are no second chances once the bucket tips.
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