I did something like this for the first time in my life, so don't laugh ;D
As I wrote in the photo riddle thread I bought that Iron Man lamp as a gift, but it came damaged. The damage wasn't big, but it was on the chest, so it was "right in your face" visible. There were also silver elements missing from the movie suit and some light details. Also the black wash was very sloppy and in general the painting wasn't great - some parts weren't even painted gold where they should be. In all honesty, I was disappointed with the quality of the Iron Man figure. It looked a bit like "zabawka z odpustu" (Poles will know what I mean). I payed over 200 PLN for this lamp and I expected something better for such money... So I decided to turn the Iron Man figure into a battle damage version in order to cover up the damage and the sloppy painting. I also added the silver elements and the lights from the suit that was used in Infinity War movies (since it was supposed to be an Infinity War suit).
I gotta tell you... Painting on "raw" plastic was EXTREMELY frustrating and more difficult than I expected lol ;O The surface was so slick that the paint wouldn't hold... Those silver elements were the worst. I had to cover them with many thin layers of paint. Also, that figure isn't big, so I was using my smallest brush and even that brush wasn't small enough ;D ;/ If that wasn't enough I had to maneuver the whole thing - I couldn't detach the figure from the lamp and soon my neck and back started hurting, because you had to be very precise with every brush stroke.
At first I planned to make Iron Man more dirty and burnt, to make the suit look more "batlle-worn", but the paint adhesion was so bad that I resigned from this idea. Luckily it turned out that under that red and gold paint there was a layer of silver paint. It doesn't show in the photos, but thanks to this those scratches I made with precision knife are silvery and it gives the whole figure a more of a "real metal" look. So that was my way of making the suit look more "battle-worn".
In the end this whole ordeal was worth it, because the person I bought it for was happy with this "custom painted" gift, but... never again... ;D If I ever decide to paint some action figure or miniature, then I'll prepare myself properly for this. People who paint miniatures usually use a primer first which makes the paint stick to the plastic. They also use special paints. I used ordinary acrylic paints. Those special paints apparently have smaller pigment particles - thanks to that they don't make all the small details on those figures "disappear" under layers of paint. I learned a thing or two about miniature painting from this experience lol ;) And I defenitely got new appreciation for miniature painters :)))))
It didn't cross my mind to take photos before I started painting, so I used photos from that internet shop link that Feniks posted in the riddle thread for "before/after" comparison:

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