Typically, wood underlayment subfloors such as plywood or OSB will be the most common applications that would benefit from a primer. Other subfloors to consider are gypsum, vinyl, existing tile, and others. If you have any doubts, then we tend to lean towards recommending the use of a primer. If you are unsure if your subfloor is suitable for bonding with DITRA-HEAT-PS, please reach out to the Customer Support team for more information.