got it painted! well again...
after hours and days of sanding/filling/sanding/... i got tired of it and decided to paint it already, knowing it wouldn't be a ferfect finish. but at least with a glossy black paint i would be able to see the bad spots better so i could fix them. but the result was verry bad, entirely covered with microscopic scratches left by the sanding. so i sanded it all again, taking off lots of the paint, and then filling it up again and sanding with smaller grit paper etc...
now it's painted again, and the result is much better. though it's not yet good enough so i will later paint a second coat.
i didn't have the camera then, so i have no pics of those stages unfortunately.
still need to finish the flap mechanism (thank you for the counter-weight idea!)
im gonna let it bake in the sun for a few days so the paint can dry completely.
also added 2 reverse lights that will be connected to my standard reverse light, but they can be switched off in the cabin.
so the finishing stage with fiberglassing, filling, sanding and painting, wich i thought would take a day, finally took almost a week.