I am redoing a REAL GOOD TOYS General Store that had been in storage for years.  One wall had fallen off which made the front come apart but that actually made it easier to repaint.  It was a really pretty yellow.  I wanted to age the building so I did some online research to see images of old general stores.  Most of them where white, but dirty and worn. So I white washed it, which toned the yellow down quite a bit.  Now I am using chalk over the dry paint to age it.  When I get it to what I want it to look like, I will spray it with a dull clear coat.  I am going to use a spray sealer rather than a brush on sealer.  I don't want the wet brush to pick up the chalk.
    I turned the building upside down to get the darker chalk into the grooves under the clapboard siding.  You know, where all the dirt collects on a real building.  Especially in the corners and along the trim.  I am using a combination of a make-up sponge and a stiff artist brush to get the chalk where I want it to go.  The chalk I am using is a set I purchased a long time ago from a model railroad company. It is loose chalk.  You can of course use chalk that crafters use which comes in solid blocks.  With just a little scraping you can loosen up the chalk particles. It might come off just using the sponge or brush.     The colors that I have are an ocher(dirty yellow), a medium brown, a medium/dark blue, and a darker green chalk. I am going to also use a dark gray or black from my crafters chalks. You can also use chalk on plastic buildings, and miniatures such as Chrysnbon to take the sheen off.  You MUST seal afterwards or it will continue to brush off.