On my last two dirt bikes, I painted the frame rails with spray-on bedliner. It looks great and is very durable. The bedliner costs $8. I also use it on other stuff. See picture below.
Here is a shot from my KTM.
I see some wear on the WRR frame rail so I think I may go this route again.
Here is a before and after of a disc guard I sprayed with bedliner.
Before
After