The purpose of the flash coat is to prohibit too much of the solvent from being absorbed into the epdm rubber. This is to avoid any swelling of the product. This light coat that many times will dry normally dries within 24 hours assuming temps above 50 degrees. The flash coat will block any additional solvents from being absorbed into the rubber. If you do not first apply a flash coat you run the risk of too much solvent being absorbed which will cause the coatings to swell and cause additional days to reduce in size.