I finally managed to paint the thing.
I was on a schedule (prep + 3 colors over 48hrs) so prep work was minimal - and it shows.
Two holes were drilled in the deck and eyelets were glued in place as I plan on installing a pair of winches under the deck at the stern to make coupling to the tug easier.
I only had time for a single pass of filler (and wet sanding with #600 paper) and it shows.
Let's say that my barge has dents and kinks like the real deal!
I went the cheap route for the paint: Kobra spray cans which are aimed at "street art" and are super affordable. 3.20 EUR/can (compared to 10-12 EUR/can for similar stuff in home improvement stores).
With the right nozzle/cap I really can't complain about the quality. I got a bunch of "calligraphy" wide caps which ouput a nice wide and solid pattern, much like a spray gun.
With the wide nozzle, a very decent result can be achieved with passes approximately 15cm (6") from the surface.
Things went wrong though: paint didn't bond to the aluminium profiles. When I took the masking tape off paint would just flake off. Aluminium surfaces had been scuffed/sanded but obviously that wasn't enough. I'll try to fix this with with a brush.
In retrospect I should have epoxied the aluminium parts back in February when I waterproofed everything instead of wiping off epoxy. It would have been a good primer.
Last step was bonding a 20mm by 5mm strip of rubber from Mobile Marine Models all around as fender.