Some friends of mine installed a whirlpool tub. The tub does not have a lip on it and the silicon keeps pulling away from the tub and wall. Which allows water to leak behind the tub and to the ceiling below. They have redone the seam 3 or four times and it still leaks. Is there a better silicon then the 100% kitchen & bathroom stuff? Should the tub surface be cleaned with anything special first to help the silicon adhere better? Could it just be a tub flexing problem? Thanks for all your help.