Part of the secret to a good repair is knowing when to stop sanding. Many novice bodymen think it is absolutely necessary to sand away every possible trace of bondo, except for in the immediate area where the repair was made. If you do that, you will actually leave a "crater" in the overall surface, which is immediately visible as soon as it is primed. When sanding a repaired area. You do NOT want to see a clear line of demarcation between the bondo'd area and the surrounding metal. that either indicates that the filler is sticking up above the metal, or has been sanded down untill it is below the surface of the metal. Rather, what you want to see, is a 'fade" from the metal to the bondo'd area, that you can not detect when lightly running your fingers over the surface, from steel to bondo.-----------Also, and I can not stress this enough---do not use a short sanding block and sand only in the area where the bondo was applied. Use a long sanding board as I mentioned in my earlier post, and take long sweeping strokes, so that the board "floats" on the surface of the undamaged metal---this will help to prevent "cratering" your repair below the metal surface.