Bathtub caulking is a long and arduous procedure. While it can be done on your own, without professional help, many people make the simple mistake of not filling the bathtub before caulking. This results in cracks that form when the tub is filled for the first time after caulking.
When a bathtub is filled with water or someone gets into it, it sags and pulls at the seams. Whatever caulk is applied to the seams needs to be adjusted for this pressure, otherwise it will crack when the bathtub is filled and the entire caulking job will go to waste. This is because caulk is not flexible. Therefore, you need to ensure that you adjust for the tension that the caulk will have exerted on it when the bathtub is being used.
Filling the Bathtub with Water
When you are caulking the bathtub, fill it three quarters of the way with water. This added weight will simulate the actual weight of the bathtub when it is being used, and will make it sag and pull at the seams. Applying the caulk at these pulled seams will allow it to dry in a position where it will not crack once the bathtub is being used again.
When handling home projects like caulking, make sure you get the small details right, otherwise you will end up having wasted a lot of time and money.