There are a number of REAL automotive products, choose the right one and use it before using the left over bath tub caulking.
Fast-n-Firm has a place, not everywhere but a place. "Self leveling" has a place, NOT everywhere (it is too runny for large gaps) but it has a place.
Use a good old 1K urethane seam sealer is pretty hard to beat in most cases. 3M #08360 is what we use at work when we are not using a 2K seam sealer which is the best but requires a gun that is about 50 bucks.
Brian
