I prefer to remove any paint I question.On bare steel put a thin coat of wash or etch primer on and epoxy it.I use PPG's DP 40.Whatever you use make sure all work can go on top of it and use the color with the highest zinc content.All bondo work,priming,sealing and painting is done on top of it.If you're at the primer surfacer stage,apply 3 coats of primer,let cure and block sand,repeat if necessary.I like to do a 180 block and a 220 block.Reprime it for the final sanding of 320,600,whatever you choose.I don't like to wetsand my primer,but if you do make sure if you hit metal or bondo to let the water fully evaporate-bondo really sucks up water and you only need to uncover a small spot for it to suck up into a larger area that you can't see.Guide coat helps alot if you have a hard time 'seeing' the primer,or differentiating between the sanded primer and a sand scratch that isn't quite gone yet.The best way to make it is pick a contrasting color (black works the best)in lacquer and thin it around 300%.Rattlecans work but they're a touch thick and load up the paper easier.Guide coat it as the last coat flashes too,then it will eat into the top and not just sit there.
