Jon

You can actually slop it a bit into the coupler box -- it's totally
plastic compatible. But yes since I file my Kadee coupler faces I've
found the blackening improves appearance as well as performance. I've
also used it on delrin stirrup steps and brake hoses. It sticks and
dries to a flat weathered black color. Unlike paints there's no fuss
or clean up and my jar is still nearly full after a year of using it.

If only we could convince Sharpie to do "weathering colors"! I've used
my silver Sharpie on a couple dozen cars so far (for galvanized metal)
and it works beautifully.

Tim O'Connor