I plan to install a couple panels for solar water heating on my roof with a foam panel on the backside. What kind of coating would be good to put on the edges and backside to protect the foam from the sun? Henry has a pure silicone roof coating that I might use on the edge but @ $65 & 12 sq ft per gallon it's awfully spendy for the 4 x 8 ft flat side.

You could cover it with window trim coil (i.e. aluminum).