The sealant around my tub and ceramic tile has turned dark and, near the front, it has cracked. I stopped using the tub so it could dry out, but I'm not sure how to proceed. I tried a putty knife, but that doesn't really work. Wondering if I should find some sort of solvent that wouldn't hurt the tub or the tile, or if it would be safer to just put sealant over everything without trying to get the old sealant up first. If I did that, my main concern would be making a good, tight, long-lasting seal. Help?