you can prolly get replacement same colour panels easier than messing around with the ones you dislike, then concentrate your efforts on the ones that cant be removed!
Hammerite the fkuckers. Sorted.
The problem will be that they were originally not painted, and finished with the normal dimpled plastic.
You need lots of coats of high bulid primer to get the bumper perfect before you apply the base coat, then laquer.
They have just not bee painted very good.
I have seen badly painted bumpers and they do look very nasty and spoil the car. I personally think original plastic bumper look better on the P38 rather than color coded anyway.