So i've been looking off and on for the last year or so for a grille with a white shell for my truck. What I finally ended up doing as a temporary solution was I got an expedition insert with a chrome shell and painted the shell white.

The shell is not flaking and is a weird off white color, i've checked my local junkyards and everyone I find was involved in a head on collision

 

Could have bought any non-chrome shell and it would have made life easier. Step one should have been to remove the chrome by sanding, then apply adhesion promoter because plastic doesn't like paint and vice-versa. Next apply a few light coats of primer, sand smooth with around 320 and then hit with wet 500. Then base coat and finally clearcoat.

I'm guessing from your post you just applied white spray paint to a chrome grille shell.

 

I sanded it with 320, primed it with plastic primer, painted it with plastic paint, and used plastic clear coat

the paint held up great for about 6 months, but was always slightly off color. That was a temporary solution that wasn't supposed to last this long =P