I know this is a little off topic ;-) but I know there are people on this forum who know a lot about this stuff and I have not had much success on other forums. The photos below show where I have tried to address rust that has started mainly on seams in the van (2006 Mercedes Sprinter). At the time I used a wire brush on an angle grinder to take the affected area back to bare steel. In one case I painted Dulux Metalsheild All Surface Primer straight on the surface and then used top coat. It has re-appeared as can be seen below.
Another time I wired brushed back to bare metal as before and then applied rust converter on the rust right inside the seam and then applied several coats of grey car primer from a can straight over the top. When dry I used a spray top coat. I know it looks rough but you can't see the roof ;-)

In both cases the rust has come back, am I doing something wrong, should I be cleaning the bare metal with something before priming. Maybe I am using crappy products?

thanks