Best primer for EPDM on damp OSB substrate?
OP
Original poster
25 March 2026 342 views
I have a job coming up where the OSB deck on a garage extension has been left exposed for about two weeks and has taken on some moisture. The customer wants ClassicBond EPDM installed.
My question is about primer selection. Normally I would use the standard ClassicBond water-based primer, but given the damp substrate I am wondering if I should switch to a solvent-based alternative or use a moisture barrier first.
The OSB is 18mm Sterling board. It is not soaking wet but definitely has some dampness when you touch it. The weather forecast is dry for the next week so it should have time to dry out a bit before I start.
Has anyone dealt with this situation before? What primer did you use and how did the bond hold up long term?
Roof area is about 16 square metres. Single storey garage flat roof with three upstands and a gutter edge. Standard job apart from the damp substrate concern.
4 replies
4 replies
Accepted answer
U
Member3d ago
I would let it dry for a few more days if you can. The water-based primer is fine on slightly damp OSB but you need to make sure there is no standing moisture. Test with a moisture meter if you have one. Anything under 18% should be fine for the ClassicBond WBA primer.
U
Member2d ago
Had the same issue last winter. I ended up using the ClassicBond solvent-based contact adhesive instead of the water-based primer. Worked perfectly and the bond was solid. The solvent-based version is more forgiving on slightly damp surfaces.
U
Member2d ago
Worth checking with ClassicBond technical directly. I called them about a similar situation and they recommended their moisture barrier primer which goes down first, then the standard WBA on top once it has cured. Adds about an hour to the job but gives you peace of mind.
U
Member2d ago
Whatever you do, do not just crack on with the standard primer on a damp deck. I made that mistake early in my career and had to go back six months later to redo the whole thing when the membrane lifted. The extra day of drying time or the cost of a moisture barrier is always worth it.
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