I wouldn't use rattle can primer - it is porus to rain. I'd go ahead and use Zero Rust, but then sand it all off (don't know how hard this would be).
I understand your desire to get 'er done, but I honestly think you're better off waiting until you can get a compressor and paint gun and do the DPLF.
Part of the delay can be alleviated by just doing the undercarriage first, which won't get rain, and use ZR on it. Not all your time will be wasted.
As far as a paint gun, get a relatively inexpensive one for primer. HF has a pretty good one for ~$49.
I predicate this last statement on the premise that after you get through with all body work, you will be putting on a coat or two of primer/surfacer and block sanding everything smooth, then a final coat of reduced DPLF as a sealer and then the final paint stages (bc/cc or single stage).
I have oe of the HF guns and bought the 3 gun set from Northern tools on sale for $89. The 3 gun set has a detail gun and two different guns, one for primer and one for paint.
These are all HVLP guns and my $400 compressor from HD handles them fine in my testing - at least at the rate that I will be spraying.
I will not be using any of these for the top coats, just for primer when I get around to actullay not dismantling and welding. Running a restaurant leaves little free time....
I like going to the AutoBodyShop Forum and reading the posts on painting there, since the last time I painted was 30 years ago.