I am almost finished stripping 15 years of varnish off the trim of my boat. Most is in good shape, but there are grey spots where the varnish was worn away, and some black areas from water intrusion. I know teak brightener will help, but I am worried about what it will do to the nearby metal and fiberglass since it is an acid. Is it safe to use, or should I just sand it all down? Any other ideas? Once complete I will be finishing it, probably with Cetol, Thanks.