I have some used hot water solar panels that have been out of service for 5 years( drained sitting on the roof).  I have rebuilt them, but when I was flushed them out I got a bunch of gunk out of them.  I flushed them out until the water ran clean, but I am assuming they still have some baked on anti-freeze on the inside of the pipes.  I would like to get the inside of the collectors as clean as possible to have the most efficient heat transfer.  Does anyone know of a liquid I can use to clean them out.  Would boiler cleaner work, or some sort of mild acid? 
Thanks,

If the insides of the panel are steel, you can use a solution of vinegar. That is how you clean your coffee pot. If the deposits are from the antifreeze solution, try the stuff that you use to clean your car radiator. Maybe this will help.