Do you have to mix hardener with your paint? If so maybe try putting a lot less in.
I've painted quite a few old tractors and parts with pretty good success. Sounds like your prep is right. I usually use the air gun to blow dust away, then degrease spraying onto a rag then wiping - don't spray onto the metal then wipe.
I've never used acid etch so I give a couple of coats of primer waiting until it's just tacky between coats. The first primer coat should be a light mist hardly visible. Then I leave it as long as possible before doing anything else, anything up to three weeks.
Before putting the topcoat on I give it a very light going over with something around 1500 grit sandpaper then wipe. First coat of topcoat should be a tac coat nothing more than a fine mist. Then a normal coat or two followed by the final one. If you want a shine put the last coat on as wet as you can without it running.
You can go further like finely sanding between top coats etc but I don't think it's necessary on chequerplate as you're not looking for limo standards.
I've usually painted in the summer in a tin shed so it's been nice and warm, can you maybe get some heat into where's you're doing it?
Just my process incase it's helpful
