Have you ever envied those bulbs with the 'perfect' finish? Well, I have, and decided to try something new (at least to me - I may have re-invented the wheel!!!).
After you have smoothed the bulb with your file, and wet sanded it with #180 wet/dry, go ahead and attach it to the fin. After the epoxy or whatever you use to attach it cures, clean the bulb thoroughly with acetone. Now, before touching it with your dirty fingers, clamp the fin in a vise so the 'pointy' end of the bulb is down. Mix up some finishing (SANDABLE) epoxy, and apply it liberally to the bulb. Get your hot air gun (or wife's hair dryer) and heat the bulb 'til the epoxy gets real runny. It will flow evenly and the excess will drip off the end of the bulb (don't forget to put something beneath the point to catch the runoff).
After the finishing epoxy cures, there will be a little drop on the point - just sand it to the desired sharpness and watch out - needle sharp bulbs HURT.
You now have a finish that is incredibly smooth, and will stand up to lots of abuse. If you want to paint it, sand with #400 wet/dry, clean the sanding residue off with acetone (I like this stuff!!), then paint as you see fit.