Sashco 30381 3/8-Inch by 100-Feet Pre-Caulking Filler Rope Backer Rod, Grey

Is this good for use in a bathroom? I have a gap between my shower wall and the tub, and want to put this in, then caulk over it.

I have an open expansion joint in my driveway of 3/4"+. Will the 1" backer rod compress into this slightly over 3/4 " joint for a tight seal?

What are the benefits of closed cell versus regular backing rods? I have a few cracks between the concrete porch and the foundation.

Can the caulk be painted/