I have used roofing felt AND fender welt. Often the fender welt doesn't have enough compliance / coverage to prevent paint wearing through from friction / body flexing.

I would not use a sealant - it will not keep the water out, but it will let the water in and hold it there. Using welt / roofing felt allows the joint to breath plus prevents scuffing / squeaking / paint abrasion through and exposing the metal to the elements.