My winter project is to repaint my steel Fuji frame. I used denatured alcohol after removing the paint to get rid of all the grease. I sprayed on a coat of duplicolor self-etching primer followed by duplicolor white primer a day later. After giving the first coat a day to dry I started lightly sanding with 600 wet/dry before I gave it a second coat. I am not sure if this should be happening but some of the paint sands right off and I am back down to metal (see pic). I don't think I am sanding too hard. Even after the second coat I am having this problem. The coat of primer is nice and smooth and feels great to the touch. Do I need finer paper or do I just need to give it a very, very light sanding?