Gaz2923
I have to respectfully disagree with Big-All. Silicon caulk will stick tenaciously to both glazed ceramic tile and the baked-on enamel finish on a steel bathtub.
The problem is that nothing sticks well to old silicon sealant, not even new silicon sealant. Consequently, unless you remove the old silicon sealant completely, then the new silicon sealant will not be sticking to either glazed tile or a baked on enamel coating; it'll be sticking to a very thin film of old silicon sealant (which in turn is sticking tenaciously to the tile or tub). And, it's the adhesion problems created by that thin film of old silicon that give rise to the idea of caulking the tub with water in it. The gameplan is to put weight in the tub so that any gap between it and the tiles widens, and caulk then so that the caulk will not be in tension when the tub is in use. The problem, of course, is that this is treating the symptom, not the cause, and the water in the tub at the time of caulking is more likely to create adhesion problems if it gets splashed up onto the surfaces you want to caulk.
Here's how to remove the old silicon completely:
Go to your caulking aisle and look for a silicon caulk remover. In Canada, DAP (which is the consumer products division of Dow Corning) markets a product called "Silicone-Be-Gone". I expect you have something similar there.
1. Remove as much of the old silicon caulk as possible with a razor blade.
2. Apply the silicon caulk remover to the residual film of silicon caulk. This stuff is gelled, so apply it with the brush fitted to the cap, but then spread it with your finger. The warmth from your finger will melt it and leave a uniform coat of the stuff on the old silicon.
Silicon caulk remover doesn't dissolve the old silicon, it just causes it to swell up and get soft so that it can be more readily removed by mechanical means, such as scrubbing it off with a green nylon ScotchBrite pad.
3. After the caulk remover has had an hour or so to work, scrape again with the razor to remove most of what's left and smear more remover over the old silicon.
4. After another hour, scrape again, and this time use a nylon scouring pad (of the kind sold in grocery stores for scrubbing pots) to remove the remaining silicone caulk. (IF you have a fiberglass or acrylic bathtub, you might want to use the less abrasive white nylon pads instead.)
5. Wash the caulk remover off with water (it's water soluble) and towel dry.
6. Any residual silicon caulk still on the tub or tile will still be soft and swollen, and this step makes use of that fact. Sprinkle some portland cement powder (or any very finely ground powder, perhaps even baking flour) onto the area where the old caulk was and rub with your finger. If any residual silicon caulk is present, then the powder will become embedded in it's soft surface and remain there, thereby revealing it's location. If there isn't any residual silicon on the surface, then the powder will wipe off the tile and tub surfaces cleanly.
7. Now, use the silicon caulk remover only on those places where the powder revealed residual silicon caulk. (Normally, at this point, it's just a matter of using a dental pick to remove pieces of silicon caulk from the cracks and crevices along the joint between the tub and tile.)
I own a 21 unit apartment block, and I have to replace the silicon caulk around every tub as it becomes discolored with mildew. I find that:
A) the trick to getting new silicon caulk to adhere properly is in removing the old silicon caulk completely, and using a silicon caulk remover and finely ground powder allows a person to do that with confidence.
B) I'm continually surprised at how much residual silicon the powder proved is still there. No matter how good a job I think I've done, the powder generally proves it wasn't as good as I thought.
C) I have 21 bathtubs and each tub has a shower, so I'm 21 times as likely to have water damage on a ceiling below if my silicon caulk isn't holding well. That also means that I have 21 times as much confidence that my method of removing the old silicon completely is the trick to getting a waterproof seal with the new silicon.
If anyone has any trouble locating DAP "Silicone-Be-Gone" caulk remover or a competitor's equivalent over there, lemme know and I'm sure I can find a hardware store or home center over here willing to ship it overseas. You'd prolly have to pay for that shipping, tho.
