Tricky peice

I'm trying to figure out how to paint the grille on my lightning. It's all chrome, but i only want the parts behind the headlights and the part that is supposed to look like it's behind the grille (see pic, the gaps behind the grille, they're not open in the model)
http://www.f150world.com/gallery/RalphsFASTGallery/47182.jpg
left chrome

The headlights should be easy, i'll just put scotch tape in them and cut around the edges, then pull up the tape when done, but for the grille wells, I was thinking of just putting small drops of some kind of glue or viscous liquid in the wells, and and taking them out after i'm done. Would any glue or material come out easily without damaging the plasic and act as an effective barrier against the acrylic paint? I'm thinking MAYBE motor oil.

White glue will work fine for that, but it will be hard for you to get it to make a straight edge...