Banish the Caulk, Spare the Tub
Advice on how to remove moldy caulk around a bathtub
How do I remove the moldy silicone caulk around my fiberglass bathtub without ruining the tub's finish? —Sidney Nau, via e-mail
This job needs to be done with care. Fiberglass tubs are easily scratched, and you have to get rid of every trace of the silicone sealant because nothing will stick to it, not even more silicone.
Start by running two strips of painter's tape around the tub, one below the joint and one above it, to protect the tub's finish. Use a sharp utility knife to cut into the center of the silicone, and carefully cut up to the edge of the caulking. Now grab the bead's cut end with needle-nose pliers and try to pull the whole thing out in one piece. If it doesn't come loose, work a razor blade under one edge of the silicone, then another to sever its grip. Finally, trim away any leftover silicone bits, pull off the tape, and scrub the joint clean, first with WD-40, then with a solution of diluted bleach and detergent, to kill any mildew. Rinse thoroughly.
Once the surface is dry, run two strips of tape again and recaulk. Be sure to use a mildew-resistant acrylic-latex sealant formulated for bathrooms. It will be easier to replace than silicone, when the time comes. Here's my step-by-step instruction on How to Recaulk a Tub.
— This Old House general contractor Tom Silva