UOP

I was wondering if you guys could help me out with a potential problem. I've had these heads off the motor (L76) for about a month now, and this morning after lapping the valves and reinstalling with a new PRC spring kit I noticed that my exhaust valves have what looks to be rust on the valve tip. I tried removing it with WD40 but no luck, and I can feel it with my fingernail. I'm a little confused on how this could happen because the heads have been kept inside out of the elements since being removed. Things do happen though and I'm afraid if I let it ride it'll just eat up the valve tip and/or rocker with my aggressive .620 lift cam.

Is this something I can polish off myself, and if so what do you recommend?

This is also a used engine I purchased for my build and have never seen it run.