Power wash off what you can first, then get some steel wool and paint remover/solvent and scrub away. I learned all this the hard way, because mine was flaking bad too because I didn't prep the surfaces properly for painting.
Odd that you had that issue. I bedlined my stuff 2 years ago and it only has nicks/scrapes from wheeling. I figured I would just do it post WENT. All I did was clean the plastics with acetone...never even sanded.
I never had any trouble out of mine neither, mines been on the truck for 3+ yrs and just now finally starting to show some Knicks and chips. But then again I did use 40 grit sandpaper on mine lol. Plus the usually wash with soap and wipe down with acetone.
I used the Rustoleum spray on, and I admit I only did the soap and water wash but no acetone wipe down. Mine lasted maybe a year, and I probably didn't spray it on thick enough. I was going to try and get my plastics as close to original as possible before spraying them with Krylon Fusion
Rusto truck bed is the same thing I used on mine. I think mine has held up so good because I had to use a ton of it to cover up the damage the 40 grit did to the plastics.
I used Duplicator in the spraycan. I did everything. Bumpers,grill, sidetrim, all the bars, and wind deflector. 6 to 8 cans I think. 2 years no fading or chipping. I cleaned with acetone with no sanding.