I experimented this week with custom painted decals. My goal was to make light yellow decal stripes to apply on the tank car. So far, the experience was positive.
The decal paper first received a light coat of Dullcote to give tooth for the paint to stick on. When dry, two coat of Tamiya light yellow (a custom mix) were sprayed. When dry, a last coat of Dullcote sealed the decals. I let it dry a few days before applying to be sure everything would hold well.
Decal stripes were cut to size and dip in mild water. As I often found out while making my own decals, film is harder to slide from the backing paper because of paint thickness. I let the decals soak in water for a few minutes instead of a few seconds. To separate the film from the paper, I used my fingers to carefully detach it. I had to do it gently so the film would break apart. In fact, the paint makes the film quite durable, so it can endure quite a bit of stress.
Application was done according to habitual methods.
If I had to redo it, I would improve my technic by doing another preparation step. First, I used transparent decal sheet. Yellow is well-known to be a color with a rather poor covering power. Two things could be done: prepaint the decal sheet with white to opacify the decal or simply use white decal sheet. Since I applied my decals on black, the light yellow stripes darkened a bit. Nothing bad enough to be noteworthy, but it would have been better if avoidable.
The last thing I found out was that for some unknown reason, a stripes skrinked while setting in decal solution. I use Solvaset and never experimented something like that. All other stripes kept their dimensions. So I can't say it was because the decal film was altered by the paint. Getting rid of the faultly stripes wasn't easy. Solvaset and Future floor finish do wonders when decalling. The seams disappear, but it also means the film and Future merge together in an irreversible way. For this reason, I had to paint the bad stripe in black and decal again over it.
In the end, I'm still very satisfied with the result. The B/A two-color scheme with the roundel logo is quite attractive. Completing the car will now be a piece of cake.