First off, if you wave the search flag, tell me what keywords or better yet link me to the thread you found these answers in cuz i found diddly

Now

Knowing the bumpers have to have a flex agent in the paint
Do they also require this flex in the primer?

When wetsanding especially the clear coat, the finish will become very dull, like an old paintjob which needs to be buffed/polished to become clear and shiney
Does each coat of clear need to be buffed/polished before each successive coat in order to prevent hazing under the surface?

What are some ways to fix hardlines

no wetsanding between coats of clear, just the topcoat