[Beechy]

Preston.....production plastic parts are copper, nickel, chrome just like old school bumpers and grills. But not as heavily coated.
ABS is the only plastic that accepts plating tough enough to survive OEM impact and exposure standards. The "B" in ABS is butylene (a form of rubber) which is acid etched to create microscopic pores that the copper anchors into. Nickel sticks to copper, but not plastic. Chrome brightens nickel. The silver reflective surface that you're looking at is actually the nickel, chrome is a thin clear layer over the nickel that prevents tarnishing, otherwise in three months the nickel looks more like raw aluminum.

Platers are reluctant to accept jobbing work because production parts are jigged to expose them to the anodes in a particular array that avoids shading. Copper is less needy, but nickel shades easily. Chrome is more like a lacquer that is deposited in dissolved salts.
One dirty or oily job put thru a plant will ruin several hundreds of gallons of hot plating solution because the oil floats and coats every subsequent part going in. The heat, almost boiling, softens and dislodges oils and contaminants like coolant and wax.
Your grill prolly has stone chips that you will complain about in the finished result. On a bumper the copper is thick enough because it has to be machine polished, then even copper a second time if necessary .....can't do that on plastics. Your grill is also painted with two pack or urethane.......try stripping either of those clinical clean.....not possible.

Vacuum metalizing is PURE, DEAD SOFT 100% aluminum one tenth of one thousandth thick so not exterior rated..... one swipe of scotchbrite (or car wash) will destroy it.....some OEM interior parts are V.M. but non-contact surfaces.

Powder coat is out, so bright silver two pack is your only choice.

Your fiberglass bumper can be plated by painting a copper-rich coating then copper,nickel, chrome. No guarantees on longevity.

SORRY!