I second Allan's statement to use the same stuff on ALL sides. You want as good a barrier as possible. Different coatings expand and contract at different rates and the junction will be a weak spot. If you recoat as soon as the coating needs it , almost anything will work. You gain time by using a coating designed for use OUTDOORS. They have UV inhibitors, (gaining recoat time) and expand and contract more than interior coatings, (fewer cracks). Chevy used a black coating that was very flexable and soaked into the wood. Those with "body color" paint usually had that on top of the "tar based" paint they used.
Nuts & Bolts....
1.Clear and shiney, use SPAR Varnish
2.Easy to refinish, (also clear) Tung oil with UV inhibitors, just wipe ot on then wipe up the excess (called OUTDOOR OIL by woodworkers)scratches alot
3.Color use a good exterior house paint or for those with deep pockets, Marine Topside Paint.
I'm doing my bed this summer and will use Spar varnish. No stain.I just scored some 80 year old clear Southern Yellow Pine timbers 2" thick from a barn here in Texas. I'll have to resaw them but they will make a bed that looks great and I can recycle ,too.
