What kind of varnish or protective coating is used on a propane tank that is exposed to 14 hours of sunlight each day? I want to paint a design using only pastel colors and will need the colors to last.
I would use One Shot alkyd sign painter's enamel for your design. It comes in nice pastel colors. It is thicker than other paints so will not "crawl" when you apply it. Cover with a good automotive clear coat that has good UV protection. A couple rattle cans from the auto supply store should do the trick.