Fuel stains around outside of fuel cap?

I noticed some fuel stains on the paint around the fuel cap one day. I didn’t think much of it – maybe that I had over-filled the tank and it had sloshed out the filler neck while driving, so I cleaned it off.

Later, I parked the car for 2 weeks under a car cover. Not only did the stains return but the car cover was also soaked in that area. The tank was only 3/4 full at that point.

Seems like pressure might be forcing fuel back out the filler? What should I be looking for?

Car is a ’93 Mk1 Golf/Rabbit Cabriolet (1.8lt, injected).

Might be a related issue, but the car also is having trouble starting – feels like fuel is not reaching the cylinders very quickly and that it needs to be primed for longer.