If you are going over a factory finish dont waste your money on priming the entire bus, especially a cheap laquer primer like duplicolor. It dries fast which could actually cause you more adhesion problems when you would have had none. Go to an auto paint supplier (see if Finish Master is around you) and get a quart of gray epoxy primer and the activator. Use this over any body filler (you can shoot it on before and fill over it also) and bare metal that you have. It is very durable, adheres very well and paint also adheres well to it. Self etching primers have an acid in them which can cause problems with certain things being applied over them, especially fillers. The worst thing you could do is strip a good factory finish, that is about the best paint base you will get. Just lightly sand the entire thing with 320, wipe it all with a wax/silicone remover, tack rag it, shoot the roof first , then come back and shoot the rest with your paint of choice. Buy a cheap palm orbital air sander and a roll of red 3m stik it paper (320) for it, its alot better than doing it all by hand.