The barns at the top are the most recent, probably done within the last eight years as something I taught in an intermediate class lesson. The farther down this
post you look, the older the paintings are. 
The gray barn with the two white silos and fence posts was done as a result of reading an excellent, inspiring book by Valfred Thelin regarding pouring on paint and letting it run. Somone's since borrowed my book... the price I pay for loaning them out..... I still have hopes of seeing it again.
There's an old mail pouch barn near us that HAD to be painted, of course, (see 2nd painting.) The late evening sunset was done before they moved the barn and put it
on a concrete block base - looks ugly now, but it surely is a relic from the past.
Some of the barns came out of my imagination while some are from photos I took. The weird (and profitable) thing is that all barn pictures sell well here in the midwest. And there are lots of them, some much better than others.Talk to an artist who's been painting a while, and they will roll their eyes about painting barns. I will too, probably, because I realize now that it's not the barn that should be painted, but the feelings we have about 'old' things like barns that need to be expressed in our paintings. I've done a couple of more modern, abstracted barn paintings that were fun to do. They took the longest to sell.
Fortunately, I'm not counting on sales to survive, so I no longer care to paint the things that I think might sell the best. I can paint what I want to paint, what interests and challenges me. And seldom is it barns anymore.Back when I got excited about getting commissions to paint, a woman asked me to paint her grandparent's old homestead, the one here with the big white house, oak tree, garish green pines, and two distant barns. After it was done, her siblings all wanted a copy. 
Now days, with the giclee' process, that would've been easy to do. But seventeen years ago, I had to repaint the same picture, with a few minor 'memory' adjustments, for all four of her siblings. GROAN... or so I thought. I found out the value of challenging myself to make it a better painting every time. Each painting was a full 22x30" sheet of paper, and I charged $129 each. YIKES. What was I thinking? But, maybe I got the best end of the deal, too, considering the lack of mature composition, garish green trees, awkward oaks... 
Enough about barns. And thank goodness, they're are all sold.
Big barn above with mountains was done when I attended my first workshop in Estes Park, Colorado with Tony Couch. What a great week with gorgeous surroundings. And as he explained, we can't really paint barns. We can only paint shapes, textures, colors, lines, values, sizes, and direction.
Left barn is a scene I usually teach in my beginner's classes.

This was painted about 8 weeks after I started a beginners class with Suzanne Mayes Wentzell in Kokomo, Indiana. Nice fog but what pathetic trees:-(
THE END! (Transparent Watercolor used in all these barn paintings.)