It depends on what type of Waterslide decal (inkjet vs laser) and type of Clear. Last waterslide decals I did were laser printed and used PPG two part CC. Even with that said, always good to test it with the clear you are going to use prior.
If planking is excessive I would recommend getting down a layer of clear, make your playfield touchups, apply waterslide decals, and clear over that. If you mess up on a playfield touchup or decal much easier to remove the fix if there is a layer of clear already down so you don't further damage the worn playfield.
