Your subfloor surface determines how your tile flooring will perform after installation. Because most peel-and-stick tiles are flexible, bumps, holes or gaps will show through the floor. A smooth, clean surface that is primed ensures the adhesive on the back of the tiles forms a good bond that won't fail or cause tiles to lift or bend.
Reasons to Prime
While your flooring will initially adhere to an unprimed subfloor, surfaces like concrete and plywood are porous materials. Peel-and-stick tiles have a relatively thin coating of adhesive on the backing, and this adhesive cannot fill the pores. When the adhesive cures, gaps may occur between the subfloor and tile. As moisture moves into the subfloor material, it may fill these gaps and lead to adhesive failure. Tiles may bubble, peel or come off entirely.
Peel-and-stick tiles can be applied over dry concrete subfloors, but it's never recommended because concrete is a porous material that allows moisture to pass through. Priming the floor with a concrete primer before installing the tiles ensures that the moisture naturally present in concrete doesn't come through to the surface, affecting the adhesive bond and damaging the tiles. Most peel-and-stick tile manufacturers will void the warranty on floors laid over unprimed concrete subfloors.
On quarter-inch plywood underlayment, primer seals the wood and provides a smoother surface for tiles to adhere to. Peel-and-stick tiles installed on unprimed plywood may lift because the adhesive isn't bonding to the surface and air pockets may form. Primer will also help prevent discoloring of your floor caused by knots or stains in the wood.
Types of Primer
Often the primer used for peel-and-stick tiles is thinned for the first coat, so it fills the pores of the subfloor surface, allowing the adhesive on the back of the tiles to form a tight bond. The second coating is applied full strength. Subfloor surfaces are usually coated with latex primer instead of oil, so they dry to a flexible finish. This allows the material beneath to breathe and expand or contract as moisture moves in and out of it. Oil primers may crack or peel with expansion and contraction of the subfloor material. Typically, peel-and-stick tile manufacturers list recommended primers on the tile packaging. If not, ask your flooring retailer which is best for your subfloor material.