Nix on borax as well. I get skins in from friends every now and then, I have found you can't always tell if a skin has been salted or boraxed. Bottom line, they just won't stick on a bows back if they have been treated.
Last time I encountered treated skins, I was deep into gluing a set on the back of a bow and noticed things weren't going right. I removed the skins, washed the glue off the back of the bow and headed to the laundry sink with the skins. I washed the skins and washed and then washed them some more. I washed the skins for at least a half hour.
My next attempt at backing a bow with them went just fine.
I later found out the skins had been salted, I couldn't tell it from looking at them.