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I noticed after years of usage, there are some cracks in my bathtub (pictures attached). I am not sure how bad they look and whether or not if I need to fix those cracks? and if I do, how can I fix them by myself if it is easy?

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I'm not sure where some of those cracks came from, but it doesn't look like anything a good quality bathroom caulk won't fix. Here is the big secret to caulking anything in the bathroom:

Clean those surfaces as good as you possibly can. Like cleaner than you've ever cleaned anything in a bathroom. After its super clean, clean it again with alcohol to make sure there is no residue at all.

Once it's clean, use a high quality silicone bathroom caulk (don't try to save a few bucks - you're buying one tube). Let it dry properly before use and enjoy a leak free tub.

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    Be certain to remove every single bit of the existing sealer or you'll have issues. Silicone sealers don't stick well to already cured Silicone – Ring Aug 28 at 18:39
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    I can only stress again: clean, clean, clean and let dry. – JACK Aug 28 at 20:40
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    Another tip is to fill the tub with water prior to caulking it, let the caulk dry, then empty the water out the tub. – Notts90 Aug 29 at 12:12
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    @Notts90 So that the tub is in the most-stretched position when applying the caulk, correct? – Paul Belanger Aug 29 at 12:25
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    @PaulBelanger correct, better to have it compressed when empty rather than stretching and detaching when full. – Notts90 Aug 29 at 12:27
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You can fix this, potentially fairly, easily. Get some Silicone caulk from your choice of hardware store. Then read the directions which will probably say something like clean the area to be applied and apply a bead of caulk.

Here is the potentially part: depending on how long the crack has been there and what material was used there could be water damage/mold behind the shower/tub. Fixing this can be time consuming and expensive. From the pictures this doesn't look like it should be an issue but depending on how long they've been there it is a possibility.

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    Also: the instructions will say to remove all previous silicone. What they mean is remove ABSOLUTELY ALL AND EVERY SINGLE LAST BIT of old silicone, since already cured silicone is like grease when you want to put water on something, it almost repels it – Hobbamok Aug 29 at 10:05
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Several people have mentioned "silicone caulk" - there is a little more that can be added to that recommendation that can make a major difference.

Silicone rubbers designed for "wet area use" typically have a 20+ year lifetime, retain their flexibility and bond well to many surfaces.

  • For bathroom use or anywhere that cosmetic appearance matters, use an SR (Silicone Rubber) that includes 'mold inhibitor'. Without this you'll find that the surface grows a thin blackish coating which is removable but which grows back. Mold seems to love SR. Products with mold inhibitor included tend to remain much the same in surface appearance for decades.

  • Use a product specified for wet area use. Other SRs may in fact be as good, but the costs are similar for a given quality level.

  • Clean the surfaces well. A light sanding will not hurt but SR will usually bond quite well to a clean surface.

  • The cheaper SR's exhibit a string acetic acid smell when setting. The released acetic acid (same as in vinegar) has a corrosive effect on specialist surfaces in some applications (eg electronics) but is very unlikely to cause problems on bathroom related materials.
    If you care about the smell you can buy "neutral cure" SRs that do not release acetic acid. They usually are somewhat more expensive but not vastly so. Neutral Cure SRs usually release small quantities of methanol when setting. While methanol is toxic in significant quantities the quantities are so small as to be unimportant in this sort of application. If you care enough just ventilate well when curing - but, again, very very unlikely to matter.

  • Silicone rubbers will bond extremely well to most common surfaces. There are some materials that they are less happy about" but even with these the bond will usually be good enough in this role. Manufacturers provide compatability guides.
    For REALLY ornery materials you can buy primers which improve adhesion - I've used SRs in many many applications (u=including commercially) and never needed to use a primer.

  • SRs will almost always hold their shape in a bead as soon as put in place. I have seen a very few specialist SRs that will run initially on a vertical surface. This is unlikely with household application targeted products.
    Setting time is typically around one day.
    SR's will skin over in 10's of minutes.

  • SRs usually set by absorbing moisture from the air. Higher humidity results in faster setting times. Nobody ever states it but if you wipe the surface once well skinned over with a damp cloth or spray with a fine water spray it will probably maximise setting rate.

  • Setting rate slow with thickness - material further from the surface sets slower. Manufacturers usually specify not more than about 4mm thickness from surface.
    My experience shows that given enough time vastly greater thicknesses will set - a whiole gun-tube will set solid via moisture ingress via an open nozzle given enough time :-) :-(.

  • If you are using a "caulking gun tune of SR and you want the unused material to not set in the tube you MUST eliminated mositure or humidity from around the tube. I take off the screw on plastic nozzle, add a layer of plastic food wrap and screw the nozzle back on over it. And THEN I wrap the whole tube in a plastic bag wity air sucked out twist and securely seal the opening with rubber bands. Anything less is liable to result in a shelf life of weeks to a month.

  • Quality: There are good, OK and terrible products "out there". Good products usually have a density very approximately the same as water - rough only. VERY heavy products have probably been "filled" - probably with chalk / calcium chloride. They may still work OK but any savings are risky. Very light products may be filled with ??? . I've met both.
    Any well known well respected manufacturer is liable to provide an OK product. Without it being meant to be an endorsement, I've had extremely good results industrially from Dow Corning ans Shin etsu products (both are among the top few largest manufacturers). But there are numerous other good brands.

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FWIW - I'm in New Zealand, manufacturing experience with Srs was mainly in China, and I've looked through numerous US product data sheets.

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Honestly, this is something you'll deal with periodically anywhere you have a caulked joint. I caulked my son's bathroom about 18 months ago and some of the caulk is starting to break down from regular use. You can use a basic latex caulk, but they just don't last as long. A couple of things to consider

  1. Pure silicone. It's a MAJOR pain to remove, smells terrible, and is mineral spirit cleanup only, but lasts a long time. The same can be said of an outdoor grade caulk.
  2. A siliconized latex caulk (like this). Don't buy the cheap stuff
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Is it a crack or a seam where the tub base joined to the side-walls? If it's the latter, most likely there is a 'lip' attached to the tub wall, extending up about a 1/2' from tub acting as a 'dam', designed to keep water, running down the shower walls, from seeping behind it. You might be in better shape than you think

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