Paul, sorry I missed your question.
If your red alkyd primer is compatible with what you plan for the top coat, then by all means rock on!
Weld through primers for the most part (there are always exceptions..) are a misnomer in that you don't weld through the paint. Just like any other paint in the direct vicinity of the extreme heat from welding, the paint is going to liquefy and outgas, in most cases causing the dreaded volcano or ballooning you see of a porous weld. Just like any other paint, the weld through should be removed from the immediate weld area.
Secondly, I read an automotive restoration publication many years back that did a comparison testing between epoxy and weld through primer. Their results had the epoxy showing better long term protection. Epoxy is the mixing of separate components that cross link. In my mind (insert jokes here) there is no way I see something non-catalyzed coming out of a spray can (the form of most weld throughs) as a better product than epoxy.. All that aside, ANYTHING is better than the bare steel most of these hidden areas had from the factory. For the most part I'll spray/brush/roll epoxy onto a part and after curing clear off the paint from the plug weld hole areas to eliminate the issue with porous welds. If you've had a chance to look at the recent updates on the Biederman fenders we've been repairing in the shop (link in my signature) we actually used NO primer between the lapped seam so there would be less chance of degradation of the plug welds, and then after any grinding etc was done, we used a brush to apply some SPI epoxy. By dragging the brush across the flange edge, it wicked the paint between the seam of the two panels. When we had paint coming out the other side, we should have good coverage. Epoxy is going to get blocked out regardless, our intent here is to prolong having to make that same repair again any time soon. Granted, this may not work in all situations, especially when the part isn't portable like the fender we did. So use the process that works best for the area you're working on.
