I have a shower that the original tiler grouted very shallowly. That is to say the grout lines are deeper than usual. They could really do with another layer of grout to bring them closer to the surface. Can I just clean all the surfaces up and grout again over the top? Will it stick, new grout to old (but barely used)? The shower broke so has hardly been used. (it's getting fixed) So the original tiles and grout don't have loads of calcium build up on them.