Welcome.You're in a slight jamb here,you need one more primer (epoxy) or lose the etch primer.You can clean all the oil off and sand or grind the steel and then go with bondo but you won't need the etch primer because you can't mud over it.This isn't the best way to do it either.Ideally you should clean the steel,sand it,etch prime it and epoxy it.Then you can mud over the epoxy.What you can do is trade the etch primer for epoxy,and it will stick just fine to the steel as long as it's clean and sanded with something like 80 or 120 grit.Bondo will kind of stick to the surfacer but you'll never get it to let loose from the epoxy.In PPG's line the green DP40 is the best over bare steel because it has the highest concentration of zinc compared to the others,and don't use white over steel.After the bondo is final sanded use the surfacer to fill the scatches and block it out.I usually finish my bondo off in something around 150 grit,block it with 180-240 and reprime it.Sometimes I'll reblock it with 240-320 and reprime it.My final sanding is 400 dry on a DA and by hand or 600 wet but I don't really like wetsanding,let alone over porous material like surfacers and if you sand through to the bondo it really soaks up water and takes a while to dry.After this sanding I seal and paint it.
