Likely your biggest enemy is dust. I used to paint a lot of my car's engine or brake parts and the more care I took to prevent dust, the better the results. I always had a dust free and clean garage, but I would always clean and remove all the dust I could before starting out a project. Then I would use cardboards and a table and create my "paint booth" which I kept immaculate until I was finished with the project. I covered the paint booth walls with wax paper, then carefully tape off my part so paint doesn't go where it is not supposed to and removed any dust again and then primed it with several small coats or quality primer, let it dry, remove the wax paper, cleaned down the area, added fresh wax paper, painted several thin coats, let dry, repeated the cleaning and new wax paper process, sprayed on the clear coat layers.
Patience and several thin coats gave the best results. Clean out the tip of your spray can after each use (hold it up side down, spray until no paint exits, about 2-3 seconds).
You know, if the celluloid is in 1/2 good condition, all you need to do is buff it, yes? Now if you want to make it all fancy, that's something else.
That German video was frustrating... Anyone that can spend 12 minutes of a 20 minute video on taping, is just talking because they love the sound of their own voice... lol.