Dont use Aircraft stripper on those Urethane/Plastic parts or you will be in a world of hurt.
To sand them smooth your better off starting with a higher grit sandpaper.
While a lower grit such as 60 / 80 etc will work you risk gouging the surface.
I use 180 to 220 to start, then work up from there typically to 320.
For priming the parts I use this product below, I am sure others will also work.
It has a flex agent built in. You can use this for the whole car so there is no
need to have different types of primer for a particular job. Its basically 3 products in 1 depending on how its reduced.
XTREME #5425 is formulated with a hybrid of epoxy and acrylic polymers, which provide excellent adhesion, good corrosion
resistance, productive dry times, and ease of sanding.
Its made by 5 star autobody products.
I made the mistake of using Aircraft Stripper on a Urethane based bumper cover one time, It sure did get rid of the paint, but it also started to melt
the bumper too !