I was warn about painting blocks that had been run with oil in them to clean them thoroughly with solvents and then brake cleaner and finally use a heat gun to dry the blocks out before painting them. That works well
You need to hold the heat on the block long enough to see the moisture come to the surface and then evaporate before moving the heated spot around to dry the complete intake valley and the front of the blocks were the timing sets go
I don't use Glyptal now, I use Rust oleum Damp Proof red primer to seal them