epoxy it first dude, the "grind paint off before applying bog" thing is an old school technique for the following reason. people put bog over bare steel because back in the old days before 2 pack paint systems were the norm when lacquer and enamel was used if you bogged over it and sanded through the paint around the repair when sanding the filler or feather edging for primer the thinner in the primer and subsequent layers of paint would attack the paint edge and youd get a nice fried up ring around the repair when it got painted. nowa days with 2 pack paint systems this isnt nessessary depending on the size of the repair you just key up the paint already there-and if the paints sound there aint much point in removing it-and some manufacturers recommend this or priming steel first over bogging over steel because the modern bogs stick better over primer than over bare steel and they dont chemically adhere the the steel . also when a panel beater grinds the paint off after getting the dent out the air has already started getting at it and then he feels the panel up on the bare steel with his bare hand (ive seen some wear gloves but not all) putting moisture on it youre already being set up for a failure. then when they put bog over the steel its sitting around soaking up moisture down to the steel untill the painter gets it to prime it and seal it off. if you have a car that has bog thats starting to lift i guarantee itll be bare steel and rust under it and youd be able to lift the rest off easy with a putty knife or screw driver under it. in the body shop doing collision repairs there isnt always time to prime it first though with insurance companies constantly putting the squeeze on times and customers demanding their cars back though and it would make it more expensive for them and joe bloggs generally want it done cheap and by the time it shows through theyve sold their beater and have a new car anyways. you cant have the customers car sitting around for a week takin all these steps. on a hotrod or something you plan to keep for a while i definately would prime it first. two medium wet coats-which is how epoxy primers are applied not like acid etch primers- of ppgs dp-40 epoxy etc (you dont want an acid etch primer) then a coat of primer surfacer you can key for the bog because dp40 is a non sand surfacer. you definately wouldnt need to grind the paint off on repairs if your car is done in two pack. if its done in acrylic lacquer or whatever definately get it off and dp40 it first