This post is longer than 10,000 characters, so I had to split it into two posts
Recaulking bathtubs is probably one thing that perplexes more homeowners than any other job. One thing that people should realize is that silicone caulk can be difficult to remove if you don't know how to do it, and NOTHING sticks well to silicone caulk, not even silicone caulk.
It's because people don't know how to remove old silicone caulk COMPLETELY that their new silicone caulk doesn't stick well, and that results in desperate advice being given, like filling the bathtub with water before caulking, which is likely going to result in trying to get caulk to stick to a wet surface which will yield predictable results.
If you remove the old silicone caulk COMPLETELY, the new silicone caulk will stick to both tub and tile like chewing gum to the underside of a church pew.
This post is in 4 parts:
1. How to remove old Silicone caulk COMPLETELY
2. What silicone caulk to buy
3. How to place new silicone caulk
4. How to clean mildewed silicone caulk
1. How to remove old silicone caulk:
The way to do this is to first remove as much as you can by mechanical means. This means scraping as much as you can off with a razor, or if you have a fiberglass or acrylic bathtub that may be damaged by a sharp razor, use a Tungsten Carbide paint scraper or a plastic windshield scraper or any other reasonably sharp but relatively soft material that won't damage your fiberglass or acrylic surfaces.
Next, apply a product called "Silicone-Be-Gone" which is made by DAP and can be found in the caulk aisle of most home centers. Silicone-Be-Gone is nothing more than gelled mineral spirits. If you can't locate Silicone-Be-Gone in your area, just buy some mineral spirits (or paint thinner or "solvent" or Varsol), get some strips of paper towel wet with the stuff, drape them over the silicone caulk and cover with Saran Wrap for an hour or two to prevent the paper towel strips from drying out.
Web page showing Silicone-Be-Gone:
(This forum won't allow me to post a URL until I have 10 posts.
Neither Silicone-Be-Gone nor mineral spirits will "dissolve" cured silicone, it will just make the silicone swell up and get much softer so that it can be more easily removed by mechanical means. After letting the Silicone-Be-Gone work for an hour or two, scrape the silicone caulk a second time and you should remove more silicone (that will accumulate on your razor blade or scraper). Apply more Silicone-Be-Gone and spread it with your finger this time so that the warmth softens the gel and you apply a thinner coat.
After leaving that Silicone-Be-Gone sit for another hour or two, scrub the area with a green or white Scotchbrite pad (or the kind sold in grocery stores for scouring pots). The difference between the green and white pads is that the green pads have an abrasive mixed into the plastic before drawing that plastic into a fiber. The white ones are pure nylon fiber which doesn't contain any abrasives. I use the green pads on my enamel steel bathtubs, but I'd probably use a white pad on fiberglass or acrylic tubs to avoid scratching the fiberglass or acrylic material.
After scrubbing with the Scotchbrite pad, clean the Silicone-Be-Gone off with clean water and dry with a cloth or paper towel. If you're using paint thinner, I'd use Simple Green to clean the paint thinner off first, and then rinse the area with clean water. There should be little silicone left, but any that is left will prevent the new silicone caulk from sticking at that spot, so we need a way to confirm that there is ABSOLUTLEY NO residual silicone caulk remaining.
Remember that the Silicone-Be-Gone makes the silicone caulk swell up and get soft. Get a small quantity (a photographic container full is plenty for one bathtub) of a very fine powder (like Portland Cement, drywall joint compound, or probably even normal baking flour would work). Use a small brush to apply that to the area where the silicone caulk was, and then rub the powder with your finger.
If there is any residual silicone caulk remaining, the fine powder will become embedded in it, thereby revealing it's location. If there is no residual silicone caulk, the powder will be wiped cleanly off the tub by your finger.
Mark the spots where the sticking of the powder reveals the existance of silicone caulk with masking tape or something, and redo just those areas again with the Silicone-Be-Gone. You should find that the residual silicone caulk is so soft now that it can be removed even by scraping the surface with a popsicle stick sharpened in a belt sander (or a plastic windshield scraper). Then, check those areas again with the fine powder.
Once the powder doesn't stick anywhere, let the area dry and start putting the new caulk on. Maybe wipe down the area where the silicone caulk was with mineral spirits (paint thinner) and allow to dry. It will evaporate completely without leaving a residue, so there's no need to clean again.
2. What Silicone caulk to buy:
Silicone caulks come in different price ranges. The more expensive caulks will contain more mildewcide, which means that the caulk will be more resistant to attack by mildew. I personally have found that Dow Corning's "786" silicone caulk is better in that respect than DAP "Titanium II" silicone caulk. If you're buying GE silicone caulk, pay a bit more and get their "1700" silicone caulk instead of their regular "Tub & Tile Silicone Caulk". Not only will these caulks be more mildew resistant, they will stick better and cure to a stronger rubber than the regular silicone caulks.
Also, Dow Corning's silicone caulk tubes have the best closure I've found. They have a threaded cap on the end of the tube. When you're finished caulking, simply unscrew the nozzle from the tube and screw the threaded cap in instead. It's also a good idea to put a small hose clamp on the end of the tube and tighten it snug before screwing the threaded cap in. This will support the tube end so that it doesn't split, and the silicone will keep well for even years this way.
However, in the last section of this post, you'll find out how to clean mildew off of silicone caulk, so you can keep any silicone caulk clean, not just the better ones.
3. How to place the new silicone caulk:
Use masking tape (1/2 or 3/4 inch wide) to mask off the joint you're wanting to caulk. Use 3/4 or even 1 inch wide masking tape if you have little experience caulking. Put the edge of the tape about 1/4 inch away from the joint on both sides of the joint. That means stopping the tape about 1/4 inch from any vertical joints you want to caulk as well. This can be done by gripping a single edge razor blade in a pair of needle nose style locking pliers. Put the tape in place and then put the edge of the razor firmly on the masking tape, and then pull the tape at an angle so that it tears off at the razor's edge.
(If you do a lot of caulking, you can probably just caulk the tub freehand without even bothering with masking tape, but I'm writing this up for new homeowners who want to get a good looking job without the caulking experience to do it freehand.)
Before putting the new silicone in place, press down the edges of the masking tape so that you're sure that the new silicone won't get under it.
Squeeze out the new caulk onto the joint and wipe it down with a dry finger so that it's relatively uniform in thickness all along the joint. Don't let so much caulk accumulate on your finger that it starts getting on the tub or tile on the OTHER side of the masking tape. At this stage, you simply want a relatively uniform amount of caulk all along the joint, and how you accomplish that really doesn't matter.
Once you've got the new silicone caulk reasonably uniformly distributed along the joint, pull the masking tape off.
Give the new silicone caulk a few minutes to "skin over". This is a good time to mix up a 50/50 solution of liquid dish washing detergent and water.