I use RAKA Blush Free and If I do epoxy work many days after an initial coat is hard, I usually wet sand the area. If you can pull your finger across the surface and see a trail, then you have blush. I know "Blush Free" supposedly means that but with the way technical literature and specs are written, I still would be on the safe side. You just don't want to do a fill coat and 2 hours later find that you have some bubbling or puckered spots. Believe me, I learned that from experience. If you wait until the epoxy is hard and fully cured, you no longer have the chemical bond.