Caulking for shower

We have an apartment that we installed a new tub and surround, anyway the surround did not fit perfectly tight around the tub and there were some gaps that needed to be caulked. About every six months we have to reseal it from the water that the shower sprinkles on it when taking a shower. My question is what is the best product to use for sealing around the tub that will not have to constantly be redone? Thank you for your time!

What are you using now?

A silicone caulking. I dont remember the name.

How big is the gap? No caulk is designed to bridge large gaps. Sometimes a foam backer rod is called for as well. Also, the tub may move when filled with water or someone stands in it, so it is often a good idea to caulk while the tub is full so the seal isn't stressed (stretched) as much when it is occupied. compression is easier on the seal.

Up to a quarter in in spots. Where can a foam backer rod be purchased?

You can probably find it at a good local home center, but you may need to look online. This link has some good info on caulking. They also explain why a backer rod helps. The company also sells a mildew free caulk. If installed properly, and you get mildew within 7-years, they'll pay a contractor to redo it. Now, what you have to do to verify it was installed properly in the first place, I did not look up. I have no association with this company and haven't used their products. But, the info on caulking is good and is based on industry standards. http://www.sashcosealants.com/Home_Improvement/Caulking_101.aspx

1/4" gap in tub deck

A 1/4" gap is huge. Does the measurement change when you fill the tub with water?

The rule of thumb for chaulking or grouting is that you want to have at the least the same depth of chaulk installed as the width. When running your chaulking along the seams you have to slow down and fill the voids to get the extra material in there.

Remove the old material and clean with rubbing alcohol and silicone the next day.

You don't want a real deep, slug of caulk, it is more flexible and less prone to failure if the ratio is more like 2:1 (if you believe the linked article). The backer rod helps in this and shaping it for best effect.