I got a chance to do a little playing this weekend. A trip to the auto supply turned up a Dupli-Color spray can in Oxford white for something to play with. I took my Porter Cable random orbit sander with 220 grit paper(the finest I could find for it) and sanded down some areas that needed touched up. The sand paper was quite aggressive right at first but after a little wear it seemed OK. After sanding and feathering I wet sanded with
320, then dried and cleaned everything. I shot some self etching primer on the areas that got to bare metal. Cleaned off everything with wax remover and sprayed a few coats of paint.After drying there was a dry area around the area I sprayed, I wet sanded that area with 1500 until it felt smooth. Then I used rubbing compound with a wool bonnet on a DeWalt 849 polisher/buffer and polished it all smooth and glossy. It's difficult to tell what is new paint and what is old paint as far as transition but the area with new paint has no chips or other imperfections.
I guess the bottom line would be that it would be better to fix it all and spray it all, just like JR said. But I was wanting to learn about blending in an area by sanding and polishing and it seemed to work well enough. That would be useful for a chip or scratch in a vehicle that doesn't need a paint job. My truck that I'm practicing on is a 92 F-350 Diesel that started as an FS truck and was then used in a landscape business, so it's scratched and dinged all over, except the couple of places I touched it up :-) The truck seems to have been kept up mechanically and drive train but the body has been somewhat neglected.
I'm wanting to get a little bondo practice with it, a little more touch up, and then I'll start trying to sell it. Then I may attempt to touch up scratches in my Ranger, it has metallic paint and clear coat and I'm sure will be somewhat more difficult to make a touch up look right.
RogerN