In the past i have cut back the primer on the repair panel/piece to shiny metal and zinc weld through primer on the bare metal of the pieces. then recoat with the zinc primer after your done and epoxy primer over the top of the repair, then seam sealer, then for good measure stonechip if its not receiving any finish/topcoats.
There is no point using epoxy if you are going to put it on top of something else not as good. The finish will only be as good as its weakest link remember..
I've been using glasurit shop primer on the between panels that overlap before welding instead of zinc spray.
I saw it first at work being tested in a salt spray chamber and it behaved quite well.
But it is useless, that's the point I am making. It isn't red oxide and hasn't been for a long long time. It is exactly the same as white and grey acrylic primers, only the colour differs. I might use it on a quick job that I don't need to last very long, but never on a quality job. Epoxy primer now seems to be the cheapest, easiest and best performing coating for virtually all substrates. With the dish I have just done, I would have had to use plastic primer on some of it. I was able to coat the lot in Lechler 29107 and know that it will outlast the customer, regardless of the weather thrown at it.
I know what you are saying, Last time we used it was about 25 years ago, Mate painted some wings with it. When he dismantled the van 5 years later. he sold the wings on, they were as good as new!