If there is fuel residue or other material on the film it can be cleaned with "Prepsol" (used in the automotive trade before painting) or acetone. Make certain you use a lint free rag to apply.

The dish washing liquid in water or the windex work really well to allow the decal, trim tape, monocote, etc to be slid into position. Make sure you remove any air bubbles.

Once dry you may like to seal the decal edges with thin CA, this way the decal wont lift at the edges after a year or two.

Good Luck,

Colin