If you go with a light coat prior to soldering that the toner will not run. If you put too much on it will become hard to solder.

Polyurethane clear spray stays stick too long and I am not sure if it will burn off like acrylic does. Regardless of what you use go with a very light coat now and a heavier one after soldering.

The trick is to spray a fog coat. Let it get tacky to hard, then another coat and so forth. The first coat will dry almost instantly and will not smudge the toner-transfer. Subsequent coats can be heavier. I suspect three, maybe four, coats done as described will give a glossy finish and not noticeably affect the toner.

Your post answered my questions. Thanks a lot.