Fair point. The 100% silicone is the best option. Honestly never paid attention to the fact it was a mix. Alex Flex is commonly available, "100% waterproof" (when not submerged), and often cheaper though.

Since the area in question is above the tubs water line it would do the job fine but you are right, 100% silicone is better so long as you don't want/need to paint the area.

Happy to help.

As one of the replies to my post said Alex Flex may not be the "best" option compared to a 100% silicon caulking alternative. That said Alex Flex is marketed as 100% water proof, so long as it is not submerged. In my experience it would work fine in this application since it's above the tubs water line but 100% silicone is better.

Edit: I'd probably go with something like GE's "supreme silicone kitchen and bath" for a 100% silicone option. It's readily available.

As others have said:

1.) Remove old caulking. razor blade and peel should do the trick.

2.) Clean the area with a blue non scratch scotch bright pad and some shower cleaner to remove any remaining caulk. (Soap and water is probably best careful of cleaner with alcohol as it will get trapped and reduce the bond of the caulk. Be sure to let the area dry before caulking no matter what you use.)

3.) Apply backer rod; also called rope caulk, or filler rope, in gap, be sure to use appropriate size backer rod for wet areas, typically silicon. Snug but not packed in. You want it to be receded a consistent distance. (For deep areas you can stuff a smaller piece of rod farther back then stick the main single piece in front of that.)

4.) Apply painters tape above and below the gap ~1/4" from area you'll be caulking. I'd suggest using 1" tape because you'll likely be using quite a bit of caulking, especially if this is the first time for you, to cover that area and when you smooth it out you wanna be sure the tape is wide enough to catch excess.

5.) Apply silicone bathroom caulking, I like 'Alex Flex" but if it's cold where you are be sure to verify the caulk will set within the temperature, especially If there is cold outside are coming out via the crack. They make more expensive caulk for colder settings.

6.) smooth out with silicone caulk applicator. You can use your finger but with large gaps you'll have better luck with an applicator.

7.) Remove painter's tape prior to drying. sometimes you can run over the caulk again with the applicator to smooth out the transition from where tape was removed.

8.) Let dry for 24 hours prior to use, lightly touch to make sure the caulk is firm and you're good to go. (12 hours maybe enough dry time but with such a large gap and winter upon us, more time is recommended.)

Hope that helps, good luck.