What can I add to Latex Paint to make it adhere better?

When I paint steel barrels I clean and wire brush them and wipe off any dust or dirt. The paint will NOT adhere well for more than 3-4 months. The paint will blister, peel and chip easily. I have to repaint every year. I need something to add for better adherence. I know a few years ago there was a additive available thru the Paint Stores, but today no one seems to know about any additives. PLEASE HELP. THANK YOU