I bought my 85 K100RS used and it had previously been resprayed -- rather badly   
When you open the fuel filler cap, there is flaking crinkle paint along the perimeter of the filler hole.  I suspect that is not the way it left the factory.  I plan to remove the filler cap assembly and then take the remains of that paint off.  It should be left bare alloy, correct?

G'day brickrider , take a look at this thread

https://www.k100-forum.com/t3719-an-extra-15-litres-in-tank-capacity

there are a couple of pics of a tank with the fuel cap assembly removed , you can see bare alloy where the seal goes between the cap & the tank , is this the bit you are talking about ?

JR

I recently stripped my tank to bare metal and sprayed it. It took the filler cap out and removed ALL paint from the entire thing. Then, when spraying, I just masked off the 'base' of the plate which the filler cap assembly screws back into, and everything else was painted.

I'd forgotten the thread you referenced, JR.  Thanks for reminding me of it. 
Based on the photos there, it appears the paint on mine was the result of the PO's respray.  The cap hinges on a ring immediately below.  That dished area is bare alloy in the photos. Mine's partially painted.  I guess we can assume that the paint that's missing is down in the tank    I aim to clean all that's left off to be certain that it doesn't follow the rest into the tank. 
Changing the fuel filter in the tank has proven to be more work than I care to do.  I've large hands....