I totally disagree... Original paint is not compliant with new finishes or primers. If you have old acrylic laquer paint on yrou car, you need to get rid of it, or seal it. acryli-seal.
If you are just filling small dings, no more than 1/8 inch deep use polyester skim coat. USC Icing, or Evercoat Thin Ice. Something to that effect...
the best way to prep things like the unnderside of a hood is with a red scotch-brite pad. use it to feather out all chips or whatever, then paint right over the old finish, or seal first then paint. but do not use a sanding primer under there, it's too much work to sand out. If you like doing lots of work for something that will only be seen when you need to get the spare tire out, or the dead body out of the trunk, then go ahead and wetsand under there, without a block, little pieces in your hand, and use red scotch-brite pads dry for cracks adn things that you can't reach with the sandpaper.
For prepping panels for paint short of stripping to bare metal, assess the damage, then scuff in the damage and 3 inches around with 80 grit or coarser, then do fill work, sand out with 80 grit, then finish sand on a DA with 180 or 220, then scuff around the area 4 inches with a red scotch-brite pad, adn primer within the repair area. mask off the rest of the panel first, leavin the repair area exposed. do not spray all the way to the masking lines as this will make a hard line that is not easy to block down. feather it. feather everything...
Then put a guidcoat over the whole panel after unmasking with a color you can see on both the primer and the rest of hte panel. use 2 different colors if you have to.
then wetsand the whole panel. use 400+ grit on a semi-hard block, not liek a block of wood, but like a hard sponge backed block, or one of those dura-blocks. something liek that.
work in long strokes and go until all the guidecoat is gone. don't bite through the primer over the body work.
then seal, or paint.
