We have a five-year-old bathroom and the tub just won't stay caulked. I have re-caulked it repeatedly, but it seems to either crack (yes, crack, despite always using 100% silicone GE II) or pull away from the wall at some point within a few months. Part of the problem is that the opening is not a uniform width. It ranges from about 1/16" to a full 1/4 ", making it hard to find the right size bead to use. If I use a larger caulk bead, there is too much overflow at the smaller sections, and it won't push into the space there; if I keep the bead tiny, it is impossible to do the larger spaces because the caulk disappears into the space. The caulk does not always fail in the same spot - it varies each time, so I don't think it's a problem with the structure. I have tried stuffing some joint-filling foam in the wider spaces, which may help a little, but not enough.
I always clean really well (including scouring and wiping well with alcohol), let it dry out for a few DAYS before caulking, (assuming that there may be moisture in the wall), fill the tub with water before I start, and work very carefully as I go. I usually manage to do an o.k.-looking job despite being a complete amateur, but no matter what, it fails way sooner than it should. Mold and mildew are not a problem, thankfully. I would love some suggestions on how to make this work. It is a cast iron tub, and the wall is ceramic tile. (Note that we first had the installer re-do it - twice - , but his job didn't last any better than mine. Also, our other tub still has the caulk that was on it when we moved in over 10 years ago, and it is sound).