The 3M 8101 or similar is used for things like bumper repair. I am finding it very close in density to the original material which helps with that difference in sanding. Adding more for filling or building up is not a problem, its glueing to glue. It is very flexible which seems less likely to chip when painted with epoxy primer which has good flex as well. I'll post some pics when I finish.

Give me ambiguity
or give me something else