If your gas cap looks anything like the one on my T34, it's full of crud and has a nasty dusty coating all over it. The original T34 gas caps are aluminum and have a VW logo on top. The replacements are gold colored and have no logo. I decided one day to revive the original luster of my gas cap, and it took only an hour and was inexpensive too … my kind of restoration work!
The supplies I used for this job included: fine sandpaper, gray primer, chrome spray paint, and masking tape.
I removed the gas cap and used the fine sandpaper to remove the grease & grime from the top & sides of the cap. I then masked off the inside of the cap with tape and set it inside my paint box (a deep cardboard box). I sprayed two thin coats of gray primer and let it dry for 30 minutes. Then I sprayed the chrome spray in three thin coats to complete the job. I went to lunch and came back to a brilliantly restored gas cap. I removed the masking tape and installed the cap.
The trick to this job is the chrome spray paint. I found a paint called Plasti-Kote Bumper Chrome enamel at my local auto store for $6. It provides the shiniest fake-chrome paint job of anything I have ever seen! It's the same one I use for restoring the headlight & fog light reflectors too, and one can lasts for a whole T34's worth of reflector paint jobs.
If you are into the minor detailing, you can use a black permanent pen to color the VW logo on the top of the cap. Make sure to wait a couple days to color the logo, however, since the paint needs drying time.
I took the route of having my cap zinc coated. I could have taken my unassembled cap to the plate shop and said "just dip it!" I know better! Zinc plating has the property of making metal harder. Springs, when they go through this process, have an tendency to become brittle and therefore break on first use. You need to remove the spring with cloth covered pliers and then you can have the remaining pieces coated. Putting the spring back in is the reverse process. Make sure you make a note on how the spring and backing plate go in!
By the way, the above picture does not do the finished cap justice. It looks terrific!