I painted the plastic grill insert on my 420SEL black and I used a can of black barbeque paint. I figures that could hold handle the heat and it did. I don't know if that comes in white. I would suggest several coats as this area is subject to flying stones etc, and the paint you put on can chip. As for the grill strips I sugggest leaving them as is, just paint the insert.

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired