Honestly I'd have no idea what they used in the 70's but i can tell you I've used everything from rattle can to truck bed coating. Best longevity was POR-15 with topcoat. Best overall for money, id go with TSC implement enamel. My Glastron trailer i shot with thinned rustoleum. Whichever you choose, be sure to leave a few drain holes to let water out Hope this helps