Bona R540 Polyurethane Primer/Moisture Barrier. 5 Liters. A one-component roll-on moisture membrane specially designed to mitigate vapor transmission on absorbent and non-absorbent subfloors. The polyurethane reactive primer is compatible with the entire Bona Adhesive System. Easy to apply- rolls on, dries fast allowing nail-down installation immediately, suited for underfloor heating, etc.