Goodness gracious, fiiiiiiinally I managed to get all the resin out. This last huge turdshaped block was still in there. I can now put almost 2,5 liter of water in the fuel tank. This is a gain of a little over HALF A LITER!
What is half a liter, you say? This means about 30 kilometers more driving! So now it's a matter of cleaning the tank up, removing all the rust, sealing it again (IF there was even a leak) and painting it.
PROGRESS PEOPLE! \o/