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hffeagan 01-02-2008 03:06 PM

Recaulking at previously silicone-caulked shower door frame
I removed previous silicone caulking which was blackening with mildew from our shower door frame where it joins fiberglass shower. :eek: Noticed on the DAP tube instructions where it warned against placing DAP white Tub and Tile Caulk at a location which had previously been sealed with a silicone caulk.
:whistling2: We had already thoroughly cleaned the area, both the chrome or stainless steel frame and adjacent fiberglass: by scraping out old caulk, wiping away all loose particles, wiping the joint a dampcloth, drying wit clean paper towel, allowing area to dry thoroughly for 18 hours; my wife wanted a nice straight edge so we (experimented) placed painter's edging tape on fiberglass and metal frame and finally applied 1/4" bead of caulk and finger smoothed it in to place.
Waited about 18 hours and began peeling edging tape from one side and promptly pulled all the caulking out. :mad:
Does anyone have experience as to what i should do to prep the fiberglass and metal surfaces before installing more caulk? Differnt brand/type of caulk? Should we forget teh edging tape? Any other ideas?
:thumbsup: Thanks in advance !! hff

Rehabber 01-02-2008 03:18 PM

Use a silicone Tub and tile caulk. It's much harder to use, but once silicone is used you are pretty well stuck with it.

Bondo 01-02-2008 03:35 PM


Cleaning Silicone Off from about Anything is Impossible,......
You'll never get All of it Off.......
The Only thing that sticks to Silicone,............. Is more Silicone.........

Obwan 11-22-2008 03:28 PM

I know this is an older thread but I am about to attempt this process. The only thing I have been told by a tradesman was NOT to use your finger to finger finish (and absolutely no spit). The finger and spit contain bacteria and can lead the slicone to grow mould. That's the reason I am attacking this job now - the silicone join is black in places and slowly disintergrating after using bleach to clean it off. I thought about wearing surgical gloves and or dipping my finger in meths. What is the method a professional would use?

Nestor_Kelebay 11-22-2008 05:55 PM


I'm making beer right now, so I only have a few minutes while I siphon from the primary to secondary fermenter.

I own a small apartment block and have 21 bathtubs caulked with silicone caulk, and I know exactly the cause and solution of the problem you're experiencing.

Will post a full reply later tonight or tomorrow.

I think every single thing said in every single post in this thread is wrong. Almost as bad as a paint thread.

PS: Don't talk to that tradesman anymore. He doesn't know what he's talking about. And, by the time you finish reading my thread, you'll know more than ANY tradesman about dealing with silicone caulk, except perhaps, a professional janitor.

Nestor_Kelebay 11-22-2008 08:44 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 on garbage advice being given, like filling the bathtub with water before caulking, which is likely going to result in trying to caulk a wet joint which will yield predictable results. Garbage in, garbage out.

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
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:

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.

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 11-22-2008 08:45 PM

"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 baking soda 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 putty knife to scoop the bleach/baking soda 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 clear 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.

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 baking soda 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). (Adding baking powder to gelled bleach results in the gelled bleach becoming ungelled so that you ruin it.) 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. I think talcum powder is less dense than baking soda, so I'm going to try making paste from talcum powder and regular bleach when I run out of paste made from baking soda to use.

Hope this helps.

Obwan 11-22-2008 09:45 PM


Outstanding. I am overwhelmed at the depth and knowledge of your response. I will let you know how it goes and can only thank you a hundred times over.

Nestor_Kelebay 11-22-2008 11:32 PM

Thanks for the Thanks, Obwan.

I use Mapei "Planipatch" floor leveler powder for finding residual silicone caulk, and it's ground to a VERY fine powder. And, of course the trick is to use a powder that's been ground fine enough to get embedded in an VERY thin film of softened silicone caulk.

I reread my last post and should mention that it'll be necessary to re-dip your hand in the soap solution occasionally as you tool the new silicone caulk. I expect most people would have recognized that.

I think if you took along a film container or prescription vial, you should find a ripped bag of portland cement or floor leveler in MOST home centers. Few store employee's would have anything against your taking a bit of it cuz they'll probably be tossing it in the dumpster or selling it for half price anyway.

Also, the monkey tradesman that told you never to use your finger to tool the silicone doesn't realize that silicone caulk has mildewcides in it to prevent mildew growing on the surface of the silicone. Your finger doesn't make that silicone any more susceptible to attack by mildew. It's been my experience that the mildew doesn't return any more quickly after cleaning than it did when the caulk was new, and that suggests to me that the mildewcide reserve in the caulk isn't depleted when the caulk gets covered by mildew. It's also been my experience that the mildew doesn't actually damage the caulk. When you clean caulk by the method outlined, it seems to look normal, like the tub was just recently caulked. NOT like the silicone caulk is deteriorated.

Finally, when cleaning mildewed silicone caulk, use regular bleach and leave it on as long as possible (more than one day). That will ensure you kill all the mildew growing on the caulk, and that it'll be pure white. If you see any beige or brown discolouration on the caulk after removing the Saran Wrap and paste, it's entirely because the paste wasn't strong enough or wasn't left on long enough.

Every single one of my tenants uses their shower, and it's normal fo the caulk to be black in places as a result. if I couldn't clean the mildewed silicone on the tubs, and had to replace it, it would be a lot of extra work for me. The success of my business depends on my being able to clean silicone caulk well so that the bathrooms look clean so that new tenants WANT to rent my suites. I've often had tenants comment on the fact that my building is so much "cleaner" and "better maintained" than they'd seen. And, unfortunately, the reason for that is that most apartment blocks are managed by care takers who don't know squat.

I saved the text of the above post to an "Answers" folder on my computer because the problems you were having with silicone caulk is a very common one among new home owners.

Obwan 01-07-2009 09:13 AM

Porject Complete
Hi Nestor_Kelebay, Sorry for taking so long to update you on the outcome. I left it until the summer (NZ). I am happy to report, hard work as it was, it has been worth it. "Looks as good as a brought one" lol. I have shared your knowledge with a couple of people at work who equally appreciate your willingness to share your knowledge. Seriously if I were you I would consider offering my notes to companies like Unilever to use as a customer "Fact Sheet". They would be invaluable.

Again many thanks

Nestor_Kelebay 01-07-2009 07:38 PM

I didn't notice your post until now, Obwan. Sorry.

You don't realize how much misinformation circulates amongst DIY'ers until you start to learn a little about home repairs. Like, for example, in your original post you said:

"The only thing I have been told by a tradesman was NOT to use your finger to finger finish (and absolutely no spit). The finger and spit contain bacteria and can lead the slicone to grow mould."

See how dumb that advice is now? Based on what that tradesman told you, he really should have had the decency to say "I don't know" instead of confusing you even more with misinformation.

Anyhow, glad things worked out for you.

Leah Frances 01-07-2009 07:50 PM

great post Nestor! I'm bookmarking it.

Nestor_Kelebay 01-08-2009 11:04 AM


Originally Posted by Leah Frances (Post 209001)
great post Nestor! I'm bookmarking it.

That's not such a great idea, Leah. The owners of DIY Q&A forums like this one will periodically "clean house" by deleting thousands of old posts. So a couple of years from now when you're wanting to replace some silicone caulk, you may find that your link points to a thread that no longer exists.

Instead, you might want to create a folder on your desktop entitled DIY_Stuff. Then copy and paste the above posts into your word processor and save the document under an appropriate name (like "Silicone_Caulk" in that DIY_Stuff folder.

That way, all the information you collect on DIY subjects is automatically presented in alphabetical order every time you open that DIY_Stuff folder, and you won't lose that information when this web site deletes it's old threads.

lenge 01-24-2009 12:06 PM

but I have a plastic tub liner...:(
You seem to know more about this issue than anyone else on the Web, and I thank you for sharing your expertise!

I'm curious, though, about the safety of using both caulk remover and the method you suggest for cleaning mildewed silicone caulk-- bleach and baking soda-- on a plastic bathtub liner. About 6 months ago I had a liner fitted into our damaged tub, and the caulk used by the liner company has mildewed badly. Before this I'd never had silicone caulk mildew on me!

I will check the scant literature that accompanied the liner installation, but if you have any tips on this issue, I'd be very grateful.

Nestor_Kelebay 01-24-2009 11:05 PM


Bleach shouldn't harm plastic. You see, plastics aren't coloured like fabrics, by dying them. Plastics are coloured by adding tiny coloured solid particles to them (called "pigments") while the plastic is still in the molten state. Because each of those pigments is encased in plastic, it is protected from the bleach. This is the reason you can use bleach on "solution dyed" carpets without harming the carpet; because solution dyed carpets get their colour from microscopically small pigments encased inside the plastic carpet pile fibers.

So, I'm not sure if I understand your question. Bleach will no more harm your tub liner than it will harm the plastic bottle it comes in. I've found that Silicone-Be-Gone WILL soften acrylic grout sealer, but that sealer hardens right back up after the Silicone-Be-Gone is removed. If you're at all concerned, simply ask your tub liner company for a small piece of the plastic your tub liner is made of, and torture it with bleach and Silicone-Be-Gone. They should have lots of the stuff kicking around the shop. (Silicone-Be-Gone is nothing more than gelled mineral spirits, so if paint thinner doesn't harm your plastic (and it shouldn't) then Silicone-Be-Gone shouldn't either.) Trust, but check.

I can assure you that mildew will grow on silicone caulk. It grows on everyone else's silicone, so it'll grow on yours. But, that growth will only happen on mildew resistant silicone caulk if it regularily gets WET, and that means someone having showers, not just baths. If everyone in your house is having baths, the mildewcide in the silicone caulk will keep the caulk mildew free for many years. But, the shower water washes the mildewcide off the surface of the caulk faster than it can migrate through the caulk to the surface. Also, not all silicone caulks have the same amount of mildewcide in them. The more expensive silicone caulks (like Dow Corning's 786) DO have more mildewcide in them than less expensive silicone caulks, so theres more mildewcide migrating to the surface of a better silicone caulk. The lack of mildew growth on your silicone caulks till now may be due to one or both factors.

Should you ever want to replace that silicone with 786 or GE's 1700, you can buy PLASTIC razor blades at Lee Valley that shouldn't cut into your plastic tub liner:,42194,40727
They come in both hard plastic and a softer, more flexible plastic.


You seem to know more about this issue than anyone else on the Web, and I thank you for sharing your expertise!
There you go. Now you know much more about applying, removing and cleaning silicone caulk than the "experts" out there answering these same questions on other web sites that are giving terrible advice like "apply the silicone caulk with the tub full of water". :no:

PS: you don't need to know the rest:

The term "silicone" actually comes from a mixture of the words silicon and ketone. You see the element silicon has 4 valence electrons, just like carbon. So, it was natural that chemists would try to make the same kinds of plastics using silicon atoms as the backbone that they were able to make using carbon as the backbone.

When they succeeded in making silicone rubber, they initially thought (because of the 1:1 ratio of silicon to oxygen needed to make the stuff) that it had a structure like this:


where the Si's are silicon atoms and the O's are oxygen atoms.

Now, a "ketone" is a class of chemicals that all have this basic structure:


Where R1 and R2 are any chemical groups and the C=O is an oxygen atom double bonded to a carbon atom, or a "carbonyl group".

If R1 and R2 are both methyl groups, then you have di-methyl ketone, which is also called "acetone", and it's often sold in small quantities as nail polish remover.

If R1 is a methyl group and R2 is an ethyl group, you have methyl ethyl ketone, or MEK, which is a solvent similar to acetone.

Scientists initially thought that the structure of silicone rubber was similar to that of a ketone so they named the new rubber SILICON + KETONE to get silicone.

So, the chemical element is "silicon" but the polymers made from it are "silicones".

Anyhow, those scientists were wrong. For some unknown (to me) reason, silicon will NEVER form a double bond with oxygen as it was assumed to. After much research, scientists found that silicones have a structure that looks like:

R1 - Si - R2
R1 - Si - R2
R1 - Si - R2
(pardon the periods) where R1 and R2 can be any chemical groups.

It's my understanding that in silicone rubber, R1 and R2 are "organic" groups, which means they both contain carbon atoms, but what those groups are, I don't know. I do know that there are different kinds of silicone rubbers, but the "no odor" variety cures by absorbing moisture from the atmosphere, and that causes crosslinking to occur to transform the liquid caulk into a rubbery material, but I don't know any more about it than that.

That repeating group:

R1 - Si - R2

is called a "siloxane" group, so if the ingredients listed on any grout or masonary sealer says "poly(something)siloxane", it's a silicone based grout or masonary sealer, and you'd probably be better off to look for an acrylic sealer instead. The reason is that nothing sticks well to siloxanes, not even siloxanes. So, if you want to re-seal grout or masonary that was previously sealed with a silicone based sealer, the new sealer (even if it's another silicone based sealer) won't stick well to the old silicone based sealer, and may peel off. With acrylic sealers, the new acrylic sealer will stick well to old acrylic sealer, so you can re-seal grout or masonary with acrylic sealers repeatedly with full confidence that the new sealer will stick well.

Ya gotta know this stuff to get your DIY'er armbadge in silicone caulk.

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