000 natural fiber brush with a good tip, unthinned paint, a steady hand, and lots of patience:
This entire depot (scratchbuilt) is hand painted with right out of the bottle Polly Scale paint. I've found that water-based paints brush better than oil-based paints, as they're a bit thicker. Don't try to add all the paint you think you'll need all at once: loading up the brush will just make the effort go longer, and paint will end up where you don't want it. Work with smaller areas, smooth out brush marks, avoid paint clumps, and don't be afraid of multiple coats.
And one tip: don't paint the trim areas red! Trying to paint white over red is an effort in frustration, even with latex paints. If you have already painted the entire structure, don't use pure white: turn it into a VERY pale gray by adding black or gray paint to the white. Black and gray pigment is thicker than white, so it covers better.
Ray Breyer
Modeling the NKP's Peoria Division, circa 1943
You could pre-paint some strip wood or thin styrene strips and then add them to the model afterwards. The house on the left is styrene, the one on the right is wood. In both cases they have corner posts or added strip stock to complete the corners.
The same for the doors. Just pre-paint the stock and attach.
Simon Modelling CB&Q and Wabash See my slowly evolving layout on my picturetrail site http://www.picturetrail.com/simontrains and our videos at http://www.youtube.com/user/MrCrispybake?feature=mhum
I'd take Simon's approach, too. Paint the trim before you put it on the structure, and then you don't have to worry about over-painting it.
For some things, like DPM models where the trim is part of the same piece, I use blue painters' tape to mask off parts I don't want painted. Even with cheap rattle-can spray paint, this works very well.
And don't be afraid to mix styrene and wood on the same model. I find it works very well for trim and other parts.
It takes an iron man to play with a toy iron horse.
As someone who semi-professionally scratchbuilds and paints structures for other modelers, I really wouldn't recommend prepainting structural components and then applying them to a model, for a variety of reasons:
Hand painting isn't hard. With a little practice and some patience, you can whip out paint jobs in no time. I did this DPM kitbash in about two hours, while watching the tube:
For the trim I cut down a smaller brush to about 1/8 inch & just take my time.