A few years ago I painted the rear bumper on my BMW 1 series with spray cans of plastic primer and Cellulose primer over the top of that. I used the primer to fill sanding scratches from previous paint removal. I kept removing the bumper from the car to work on it on a stand. The car was constantly in use so the bumper was always on and off it lots of time’s in between me working on it. In order to remove and refit the bumper I had to flex the sides a little for them to clip on to the bumper retaining clips on the car. After some time of doing this the paint cracked along the areas that required bending to remove and refit. PP-EPDM seems to be an awkward material to work with. I tried using a bumper repair filler and it broke off every time I went to sand it. It did this even with keying the surface with 40 grit. In the end I had to make up my own flexible filler out of UPOL quicksand filler mixed 1:1 with UPOL Top Stop Gold polyester body stopper. It has stood the test of time. Ive seen a 4:1 high build cellulose primer for sale and it’s said that it works brilliantly on the plastic bumpers of TVR’s. I’m very reluctant to use this now even though it would give brilliant filling capacities compared to cans of plastic filler primer. If I had an air fed mask I would use 2K epoxy, 2K plastic filler primer or something like that. I know that stuff is suitable for plastics. So, Did the cellulose crack because 1. It is not flexible for Polypropylene PP+EPDM rubber bumpers. More suitable material on TVR bumpers? 2. Me taking the bumper on and off repeatedly meant the paint never had time to properly cure and strengthen? If so will an infra red lamp solve this? Do I leave the bumper off or on until I’m ready to basecoat it? Thoughts? Thanks.