Wood treatment and GRP

No idea but in no professional opinion you shouldnt seal the deck prior to fibreglassing it would make sense it would stop it sticking to the deck and causing delimitation. I’ve never heard of sealing a deck prior to fibreglassing I might add too. You were better off taking the ply down or it that isn’t an option putting a large tarpaulin or thick plastic you get on a roll that usually black and used to line floors prior to concreting. To be safe I’d flip the boards over and make sure they are proper dry before laying any fibreglass on them. Also the advise against using plywood for fibreglass and use 18mm OSB tounge and groove as then you haven’t gotta worry about leaving a 10mm gap around all edges of thenboards for expansion and contraction failure to do this will mean the edge of all the boards might curve slightly adding stress to fibreglass which might cause it to crack and fail in due time. Hope this helps

Plywood should not be used on any roof deck neither should any straight edge boards. Even on a dry ply deck with G4 the laminate can fail and de bond since it does not readily adhere to this substrate nor lead. Worse case you could have sanded down and heavily scored the ply but I have to agree with BT roofing that you should ideally use OSB T&G for GRP & EPDM installation.