I got a little bit further with cleaning the fuel tank. Every time I rinse it out, there still appear to be pieces of resin left behind. It is currently undergoing its third wash and I really hope that this will be the final one because the solvant is quite horrible stuff. It is also quite bad if you get it on your paint, but the manual clearly states this.
So the third wash also includes nuts, bolts and screws to shake around, hoping that i can get everything out now. Maybe it was better to have bought two bottles of solvant, but who could've guessed that this tank was so messed up?