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Christian 01-26-2009 02:09 PM

bathtub caulk
Hey all...probably an easy one for you...

I've tried twice re-caulking an old bathtub around the top side of the tub where it meets the tile, and twice the caulk has cracked / worn away / "melted" away.

The caulk I'm using says for bathtubs right on the tube...I think it's the right stuff.

If anyone can let me know what the problem might be it would be helpful. Also, is there a general cure time for caulk?



glennjanie 01-26-2009 04:47 PM

Hello Christian:
Tub and Tile caulk usually is silicone or, at least, silicone based which should work just fine. You may want to try using a pure silicone white caulk in the traditional round tube for a caulking gun.

Clean all the existing caulk from the crack, trim the tip of the tube to the size of most of the crack, place the nozzle on one end of the crack and point it twoard the other end, begin to press the trigger pressuring the caulk to the bottom of the crack and advancing the tube as the caulk just pushes out larger than the crack or nozzle opening. The caulk can be tooled at once with a round piece of flexible rubber hose or your finger. Be sure to have lots of paper towells handy to keep your finger clean. The caulk will cure in 24 hours and there should not be any showers or use of water until the time is up.

Christian 01-27-2009 02:25 PM

Thanks. I think the problem might have been the cure time...I'll give it a try.

Johnboy555 03-19-2009 03:20 AM

If there was mildew behind the old caulk that has to be "killed", or the new caulk won't seal. Cut out the old caulk with a razor scraperand utility knife. If there is mildew, use some regular bleach and hot water (1/2 & 1/2) and spray it in there. If it's a porcelain tub get some MEK (methel ethyl keytone) in the paint dept at any hardware or big box. this is mean stuff, open windows and use sparingly, to clean off old silicone. Let dry and recaulk using a good grade of silicone bathtub caulk. The "bathtub" caulk has a mildew fighting ingredient in it.

GBR 03-24-2009 12:52 PM

If the new caulking also separates, you might try this. Start over, cleaning,etc., preparing for new caulk, then........ fill the tub with water, with your added weight in it, then caulk. The sub floor would possibly lower to the load point that keeps breaking the caulk seal. Obey cure times as was stated above. Be safe, GBR

Nestor_Kelebay 03-29-2009 11:45 PM


I think I can assure you that the problem isn't how you caulked or how long you allowed the caulk to cure. The problem is that you didn't actually apply the caulk to the tile and tub, you applied it to a microscopically thin layer of silicone caulk that remained on the surface of the tile and tub that you didn't remove.

Silicone caulk sticks to both glazed ceramic tile and the porcelain enamel surface of a steel bathtub like chewing gum to the underside of a church pew. However, NOTHING sticks well to old cured silicone caulk, not ever new silicone caulk. And, not removing the old silicone caulk COMPLETELY is what's doing you in. There's a remaining film of silicone caulk that's sticking tenaciously to your tub and tile, and the new silicone caulk is not sticking to that residual film of old silicone caulk.

Here, lemme copy and paste a post from a different DIY forum I used to post on. If that post doesn't answer all your questions, post again and I'll try to answer them.

PS: I own a small 21 unit apartment block, and each apartment has ceramic tiling around it's enamel steel bathtub. That means I have to do this kind of work 21 times as often as the average homeowner with a single bathroom house. That means that the silicone caulking I put on has to adhere properly every time I caulk. Otherwise I'd have my hands full just replacing the silicone caulk on all of my bathtubs. And, I wouldn't have time to do all the renovating I do in my building. :) And that means I have to know what I'm doing when it comes to silicone caulk.

Nestor_Kelebay 03-29-2009 11:58 PM

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. :confused:)

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.

Nestor_Kelebay 03-30-2009 12:04 AM

"Tool" the new silicone caulk with the diluted soap solution. The soap in it will both act as a lubricant and will prevent the silicone from sticking to your finger. Basically the procedure is to dip the ends of the thumb, forefinger, middle finger and ring finger of your working hand in the soap solution, rub your fingers together, and wipe a 6 to 9 inch length of the new silicone caulk fairly lightly to give it a concave shape. Then, rub your finger tips together again to reload your working finger with more soap solution and do another 6 to 9 inches or so. Keep going, rubbing your finger tips together to soap up your working finger tip. As you do this, soap will come off your finger and coat the silicon caulk. After you do the whole joint, you can run your finger from one end of it to the other just to ensure it's more or less uniform.

Also, at this point, if you wipe the silicone too hard and remove it from one area, you can simply add a little more caulk to that area and spread it as best you can. I can't explain why it sticks with the soap solution on the existing caulk, but it does. If you put on too much, scrape off the excess with a putty knife or something.

Allow the silicone caulk to cure overnight and you can shower the next day. Don't put any shampoo bottles near it for the first 2 or 3 days, tho.

4. How to clean mildewed silicone:

Mix up some talcum powder (magnesium silicate) with straight bleach to make a paste that you can spread with a tea spoon (even a plastic tea spoon). (I use a small oval painting spatula that you can buy in art supply stores for painting with thick "paste" style paints that come in tubes.)

Use a big soup spoon to scoop the bleach/talc paste out of the mixing container and hold it near the silicone caulk. Scrape the paste onto the silicone caulk and spread it with the teaspoon.

Cut Saran Wrap (or any clinging food wrap that will stick to tub and tile) into 4 to 6 inch wide (or so) strips and put them over the paste so it doesn't dry out, and leave it overnight like that. My experience is that it won't do any harm to ceramic wall tiles or enameled steel tubs to leave it for several days like that. (I do this often enough that I just cut a 12 inch roll of Saran Wrap into two 6 inch wide rolls with a razor knife, and use those.)

When you pull the Saran Wrap off and remove the paste, your silicone caulk will be white as Manitoba snow. And, you can do this repeatedly to the same caulk to maintain it in a mildew free condition.

You'll find when you're doing this that the talc settles out of the bleach so that it'll be necessary to continually mix the paste up before scooping some out. If you're doing a vertical or joint on a ceiling, you CAN mix Vim Thick Bleach (made by Unilever Canada 1-800-896-9286) which is just gelled bleach with talcum powder to make a paste that sticks well (and behaves all the world like joint compound). However, since Vim Thick Bleach only has a sodium hypochlorite content of about 3.5 percent, it's not as effective at killing mildew than regular bleach which is 6.25 percent NaOCl.

I save the old paste and reuse it. Just add some more bleach and mix it up, and it's ready to be used again.

Nestor_Kelebay 03-30-2009 01:48 AM


Originally Posted by Johnboy555 (Post 28591)
If it's a porcelain tub get some MEK (methel ethyl keytone) in the paint dept at any hardware or big box. this is mean stuff, open windows and use sparingly, to clean off old silicone.

Can you tell me what you know about methyl ethyl ketone?

Would it surprise you to know that methyl ethyl ketone and acetone were (chemically speaking) cousins?

And, acetone is one of the safest ketone solvents. It has less deleterious effect on you humans than most other hydrocarbon solvents.

Here's how to think of it:

All ketones are hydrocarbon solvents with the general formula:

A-(Carbonyl Group)-B



If both A and B are methyl groups (-CH3), then it's called di-methyl ketone, or acetone for short.

If A is a methyl group and B is an ethyl group (-CH2-CH3), then it's called Methyl Ethyl Ketone, or MEK for short.

Thus, MEK and acetone are chemically very similar, and so they tend to behave in similar ways.

Redwood 04-13-2009 07:55 PM

My preferance for caulking a tub would be Polyseamseal Adhesive Caulk.

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