I am not aware of any additive that would stop the leak.
The bolts/screws should come out with care. The trick is take your time, use of heat may well help if they are tight. Worst case scenario is the heads break off. If this happens all is not lost as it may be possible to weld a nut on the remains (the heat of doing this often helps free the bolt). Last resort is drilling them out. All part of the fun and challenges of our hobby.