Hate to spoil the party, the best thing you can do now is get a whole new valve.
Nylon is used in that application because it is soft enough to flow under pressure and form to your valve seat. When you pulled the stem out, it was rough because debris had gotten into the valve seat between the nylon seal and the seal deformed to them. You could have just cleaned everything up, including the valve seat, and then started using the gun again until the Nylon once again formed to the seat.
When you "light sanded" the seal, you not only changed the shape of the seal, but you also embedded grit in the Nylon. When you applied the seal against the seat again, you embedded that grit in the valve seat as well.
If you really want to save the parts, you're going to need to get some non embedding lapping compound and lap the seal and the seat together, then rinse the parts with a light solvent really, really well.
The non-embedding compounds really aren't, they're made from a friable material that gets ground up smaller and smaller. Try here
http://www.mcmaster.com/#lapping-compounds/=bh83q8.
Aluminum valves are light, inexpensive and easy to manufacture, but they really aren't rebuildable.
"Bones heal, chicks dig scars, pain is temporary, glory is forever."
-Evel Knievel
