Polyester resin does not achieve a good enough secondary bond to be applied over an epoxy-based primer for structural purposes, or for fiber reinforcement applications. If the polyester resin is being applied as a cosmetic finish, such as a gelcoat, it should have an adequate bond.
The bond strength of the polyester resin to the primer is directly proportional to the quality of surface preparation performed before applying the polyester resin. The epoxy primer must be fully cured for at least 5-7 days (minimum). Wash the surface with a mild soap and warm water before sanding the surface. This will remove any potential amine blush regardless if the product has blushing characteristics. Sanding and other solvents will not remove any amine blush. If there is any amine blush on the surface the polyester resin can delaminate from the epoxy primer quite easily. Once the surface has been washed, allow it to dry completely. Sand the surface with 80-grit (or coarser) sandpaper, remove any sanding residue. Do not use any oily solvents as solvent wipes. Wipe the surface with clean, cotton rags dampened with acetone to remove residual residue. Follow with a clean, cotton rag after the acetone wipe. Apply the polyester resin under the appropriate application conditions as directed on the polyester resin product.