Old gel coats are often porous and may have absorbed years worth of waxes and oils, a condition that reduces the ability of new paint to adhere to the surface. Thorough dewaxing and sanding is needed to make sure that contaminates are removed. This is followed by special primer coats that improve adhesive properties. All surface irregularities must be smoothed out, old holes and scratches filled and carefully faired out. There's nothing like a fresh, glossy coat of new paint to show up surface defects. Unless this work is carefully accomplished, all existing surface defects will be magnified and you will not be happy with the result. [links]