We bought some used panels and a few of them are not putting out the voltage they should. We have located a short and have tried solder, which doesn't hold on, we have tried epoxy which passes current only when wet. Anyone have a solution?

Post a foto of the fault

If you go to your local ford dealer you can buy a little nail polish bottle called defrost grid repair. Its a polish full of silver particles for painting breaks in defrost grids obviously. Extremely conductive. Just brush it on and let it dry. You can perform multiple coats if you desire. Letting each coat dry in between. I do not know if there are aftermarket ones available at the local parts stores or not, but I would suspect there is.

The after market product that Eric referred to is called conductive paint. It can have either silver or nickel dissolved into the paint to make it work.
By the way, they also make conductive glue and conductive grease.