The sanded caulk I've used was acrylic-based, meaning the base is of the same basic chemistry as a typical painter's caulk like the Alex product line.
This type of caulk does have a whole different set of characteristics, it does not apply or cure in the same way as a silicone. Acrylic caulks will "skin over" within an hour or two, but it can take quite some time for a thick bead to cure all the way through its thickness - and it absolutely will shrink as it cures. But given proper ime & conditions for curing, it should not remain "mushy" - a fingertip shouldn't dent it, but a fingernail will.
It should be fine to apply a fresh bead of sanded caulk directly over a CLEAN & recent sanded caulk application. Due to the shrinkage of the caulk, this is a very common practice.
Pro Contributor, www.painterforum.com
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