Which chassis paint ???

[rich weyand]

Rustoleum Black is a perfect color match. You can paint part of a piece like the frame rail and see no line when it dries.

Note that it takes a while to dry. The way Rustoleum works is that it stays flexible, so the paint doesn't suffer from micro-fracturing with mild flex and temperature cycling. This does not open pores in the paint for water to get at the rmetal.

That also, however, means it's a slow dryer.

If the piece has rust on it, use the rust brown primer first. It forms the strongest bond, actually incorporating the rust into its matrix. Then two coats of the black over that.