Having painted table top world war 2 miniatures, heres a suggestion..
Get some cheap gray acrylic paint and some rubbing alcohol.
on a palette mix a tiny bit of the gray with some water and some alcohol. You want tinted water, not runny paint.
Then,
with a very sharp tipped liner brush, take some of the gray tint water alcohol mix, (you don't want a drop showing at the tip, but just enough to see that the brush is holding some, you'll get the hang of it..) your going to touch the tip to the crevice of the doors, and allow capillary action to do the work for you.
You'll only be able to do one horizontal surface at a time, or it will run, but this should dry relatively quickly.
once the model is completed, hit it with a light dusting of weathing and dulcoat.
I know this is a tedious task, but the results are worth it..
If it was HO scale, I would suggest using the finest point Technical Pen you can buy. They are truly a pain to maintain at times, but the results in 1/100th scale tanks are awesome, so surely it would work in HO scale. I wouldn't suggest it in N though, the capillary method would be better in my opinion.
Joe.