I think you'll have more success if you put a couple of coats of top coat on your patches. Many primers and undercoats are slightly porous, whereas top coat should be better at keeping the water out. Going back to bare metal is always best. Relatively straightforward on flat steel, but more difficult in awkward corners, welds and pitted areas (unless you have access to grit blasting or a needle gun). These latter areas are where rust converter can help out.

Thanks Chris. Not sure my wife would let me buy something with “RANDY III +” on it though!

Thanks everyone! Sorry for the long time before I replied. I assumed I'd get notifications and I didn't. I appreciate the information. Especially the recommendations for companies.

And for decades until recently, AI stood for "artificial insemination"...