That s the by the book way of treating that joint.
Backer rod around tub.
Apply it to the wall with the bottom edge about 1 4 in.
Based on your description that when the tub is full of water the joint is less than 1 4 wide i would put in the 1 4 backer rod with the tub full of water and apply the sealant without water in the tub.
Any gaps that are wider or deeper than this should use backing material such as this caulk backer rod available on amazon to fill in the gap and allow the caulk to sit securely across the.
There should be a small gap between the backerboard and the tub.
The vertical portion of the flange will point towards your ceiling while the horizontal part will attach to your tub s edge.
Probably the simplest way is to caulk the gap with 100 silicone or a urethane sealant.
Caulk backer rod to help fill gaps and openings in pavement cracks wall joints foundations sidewalks and driveways.
They can be made of several different materials.
Clean all surfaces to you want the caulking material to adhere.
This is based on the joint being deep enough so you can apply between a 1 8 to 1 4 thick sealant on top if allowed by the sealant manufacturer.
Fill the joint with backer rod then caulk with 100 silicone.
Use a cloth and water diluted detergent to scrub.
Apply it directly onto the sealant to loosen it.
Then apply a generous amount of thin set to the bottom bathtub tile and position it 1 8 in.
The silicone joint is supposed to be twice as wide as it is deep.
The proper way is to leave a 1 8 inch gap between tile tub.
In some cases you may need an l shaped tub flange to fill in a gap between the wall and your bathtub.
Above the lip of the tub.
Use a putty knife to.
In these photos i used a hydro ban.
Make sure to buy one that is made for wet areas and that is nonporous so it will not.
How to use backer rod caulking material step 1.
Fill the gap between the tile and the tub with caulk.
There are many places around your house where energy is wasted.
Backer rods are flexible foam tubes of different densities.
Remove old sealant with caulk remover.
After you have pushed the backer rod into place apply caulk on both sides of the rod to secure it in place.
The backer rod helps before caulking use the md building products 1 2 in.