Caulk

The general issues in selecting a caulk are:

• Silicone or urethane usually performs best overall. It lasts longest and holds up best, but is harder to apply (can make a real mess) and silicone isn't paintable.  Silicone "Tub and Tile" caulk has a mildewcide -- use in areas that get wet, but don't use it in area where it will contact food or drinking water.  There are other versions without the mildewcide for these applications.   [A possible exception to silicone being the best caulk is urethane caulk which may be as good and is paintable.  See discussion below.]
   
• Elastomeric latex sealant (caulk) comes next in durability and is paintable.

• Latex, silicone/latex (siliconized) and acrylic is next,  but there's a lot of variation in quality within and between brands.

• Latex is easiest to apply and cleanup.

• Butyl rubber is dead last.

Silicone is essential to seal around bathtubs, showers, lavatories, etc. There's a special bathroom formulation for this that has a mildewcide. This poison has been known to be hazardous if used inside dishwashers because the high heat outgasses the poison onto the dishes and is then eaten.

THAT well known consumer product rating magazine last rated caulk in 1993. Here's what their tests showed then:

• You can't judge anything by brand. Every brand had caulks that scored throughout the range from good to bad.

• The best silicone caulks were GE Silicone II and Ace 50 Year Silicone, with Red Devil 100% Silicone next. GE Silicone II comes in 2 types -- one with mildewside for 'Kitchen & Bath' and one without for 'Windows & Doors'

• DAP  DYNAFLEX 230 previously called Formula '230' is the best latex.^Note1  It's an elastomeric latex sealant.  It's more expensive but, it's better than all silicone-latex caulks and better than some pure silicones. I use this caulk where silicone isn't appropriate . It shrinks a lot. I recommend you don't use the clear version unless you have to.  The clear has an additive to make it go on white so it's easy to see if you're covering cracks.  It turns clear as it cures.  This causes it to shrink a WHOLE LOT and it can't be painted for a week!  The regular colors shrink less and can be painted in a couple of hours.
NOTE 1:  I've been using and recommending DAP Dynaflex 230 caulk for years.  Now, I have to face the fact that there's a serious problem with this caulk.  It appears to be one of the worlds best nutrients for mildew.   I made this page to illustrate the problem I've had with it.  I've written DAP about this three times over a period of three-five years -- they never reply.  I assume this means they don't have an answer.  If it weren't for this mildew problem it's an excellent caulk.  It is  much easer to apply than urethane or silicone caulk and is paintable.  Urethane's paintable, but silicone is not.  Dynaflex stays pliable for years, sealing joints that expand and contract, moving apart and back together.  But, the mildew problem is a major one.  They now have a product named ALEX UltraŽ 230 Premium which they boast has great mildew resistance. If it's DYNAFLEX with real mildew resistance it will be the solution, but I've never used it. 

The 2 worst other than butyl caulks were:

.....Ace 25 Year Siliconized
.. ...Seamseal Plus Silicon/Latex

Remember, you just can't judge by brand--two of the top rated and two of the worst rated caulks are from the same brands--Ace and DAP?

Some  people on alt.home.repair swear by a caulk named Polyseamseal adhesive/sealent (another elastomeric)  They're particularly high on this version for caulking tub/showers..  Unfortunately it wasn't rated, and I've never seen this product on store shelves. 

Tips

Tip1:  Cold caulk can get stiff and hard to squeeze out of the tube.  If it's cold, put the tube it in a pan of warm water for 20 minutes or so.

Tips2:  Here are some ways to reseal partially used caulk tubes.

In the past I used a large nail stuck into the tip.  This works good for keeping the tip clear, but the nail sometimes rusts causing you to have to pump out a lot of caulk to get rid of the rust.  

Right now I'm using a large wirenut (used to connect electric wiring) to cap the tip if I'll be using it again within a month.  The gray wirenuts are about the right size.  For longer term storage I use a piece of plastic (Saran Wrap or plastic bag type) over the tip, held in place with masking tape or a rubber band.  Squirt a little caulk into this "sock" covering the tip -- it will harden first, helping seal the caulk in the tube.  Some people use a piece of duck tape instead of the plastic wrap but it doesn't seem like it would be airtight enough.

Alonso Llano <alonso.llano@juno.com> writes:
"I use a screw to seal my tubes of caulking, it's very inexpensive (actually free) and works all the time."   NOTE:  You may want to use a rust proof screw.   Some folks use pliers to pull the screw out, claiming this pulls any hardened caulk out with it. -- jim

These Little Red Caps sound like a clever way to reseal tubes of caulk, but I've never used them.  If you've used one on a tube that remained unused for as much as 3 months, please let me know whether it kept the caulk nozzle from clogging.  If you're going to store it for more than a week I'd squirt a daub of caulk into the rubber sock.

Some people say they have good luck with DAP Caps.  I've never used one, but they seem to be a cap for open tubes as well as a caulk troweling tool.  I've use my finger for troweling and have never had good luck using a tool to trowel caulk, but I've seen TV handymen suggest using them so others must have a different experience with them than I have.

Recaulking a Shower or, a Tub-Tile Joint -- Getting a good seal is harder than you think.

Here's an article on caulking your home for energy saving.