Sorry for the double posting but I'm not getting any responses in the Solar sub forum.

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. I looked at some plastic roof panels called SunBloc at Home Depot last night. I'm thinking those might work to protect the flat side.

What about using a thin coat of modified thinset? That can generally bond to fresh, roughened foam. Or use an EIFS base coat. That definitely adheres. Not sure about price on that though. It normally comes in 5 gallon pails, so it may not be practical.
Why do the backside? It shouldn't ever see sunlight

The panels will be at 60° from horizontal oriented east/west. There will be sunlight on the backside during the summer early morning and late afternoon. I think I'll hide the edge from the sun with 1 1/2" wide roofing drip edge and the backside with Suntuf roof panels.

I have used exterior latex paint that has been on for over five years with no issues.

You could also use elastomeric roof paint:

http://www.gardner-gibson.com/products/cool-roof-coatings-repairs/white-elastomeric-roof-coating/black-jack-5530-1-30