Can't rust. It's plastic.
Let me be more clear: when you do one pass with the touchup brush and let it dry, it isn't as smooth as the surrounding paint, obviously. A little application of 2000 grit wet paper (lots of water, by the way, keep the paper soft and wet), and patience (no pressure, just let the tensile stiffness of the paper apply its own pressure), you can knock back the highest points i.e. smooth the touchup out a bit. If, as is likely, you need to polish the surrounding area to get rid of any hazing caused by the sandpaper action, you'll want to be careful to avoid the touchup as much as possible; because polishing/rubbing compound gets into the touchup and darkens it. This feels like a no-win situation but you can always do it over until you say "enough of that!"

Waxing the whole thing once you're done helps make it less noticeable too, so that it doesn't give a shout out when you walk by. Hah.