Do you have tried it before.

I recently bought 5 used flat plate solar panels for 70 bucks each with the intentions of heating my unheated basement slab. I installed the panels and after running the system a few days am not happy with the results. The output temps of the panels are not as high as I feel they are capable of producing.
The panels are I think 10 or more years old, factory made, but I can find no name of the manufacture anywhere. The panels are 4x10' each, The glazing is glass with a dimpled texture, the sides and attach brackets are extruded alu., the upper and lower inlet tubing is 1"copper with threaded fittings at the ends to join them, the capillary tubing looks to be maybe 3/8" copper tubing with copper fins that are rippled . It looks as if the origional black color is thin and wearing off as I can see copper color in places through the glass.
Any sugestions what I can do to get a few more btu's out of these guys? Other than giving the inside of the glass a good cleaning, what is the best way to get the copper black, (what kind of paint, or)? And what is the best insulation to use behind the fins if it needs to be replaced.