Nestor, I'm not sure I would go so far as to call not thinning a "mistake". At least I've never seen it referred to as a standard practice, certainly not 15-20%. (Wouldn't that compromise hide by quite a bit?) Some paints go so far as to be marked "do not thin" on the can.
SirWired
SirWired:
My experience has been that thinning latex paint that much has produced good results. I agree, 20 percent sounds like a lot, but try it and you'll find you need to thin more than you expect to get good results.
About: "Do Not Thin"
Notice that you never used to see that warning on oil based products years ago.
The ONLY reason you see "Do Not Thin" on some oil based paint products nowadays is because the manufacturer's back is up against the wall, and the only way he can get his product to meet rigid VOC regulations is by putting less VOC in when making the product. The idea here is that if the manufacturer tells you not to thin the product, the vast majority of people would follow that advice and the product will remain within the VOC requirements to be legally marketable.
That is, the easiest way to meet VOC requirements it to simply put less VOCs in the product at the factory. Simple!
You have little to lose by using a brush to dip some of that "Do Not Thin" product into an empty soup can, thinning with a bit of mineral spirits and seeing if it works better or not. Chances are it will because that "don't thin me" warning is there to meet VOC requirements, not cuz you'll harm the product or how well it performs.
About thinning:
As a landlord, my job description revolves mostly around cleaning up other people's messes, paying bills and fixing nail holes. I also had problems with brush strokes over every repaired nail hole until I started thinning my latex paints quite a bit. Next time you need to do a good job with a brush, dip some of your latex paint into an empty container and thin it quite a bit with water. Apply the thinned paint with horizontal brush strokes and let them self level by themselves. Don't put on so much that the paint drips down the wall or sags as it dries; it's better to put on two thin coats than one thick coat. And, as you advised, don't over brush.
Once you discover how much better a paint job you can do with a brush by just thinning your latex (or oil based) paints, you'll be a convert just like me.
About hide:
Good point.
ALL of the thinner that you add to oil based or latex paints will evaporate out of the paint while it's drying. The result of this is that if you thin your paint, then the dry film thickness will be smaller than you'll get with unthinned paint. If you thin the paint 10 percent by volume with thinner, the resulting dry film thickness will be smaller, but it won't necessarily be 10 percent smaller because the original paint contained a lot of thinner too. As a result of the smallery dry film thickness you'll get less hide per coat if you thin your paint.
About the only time all the thinner you add doesn't evaporate from the coating as it dries is in the case of thinning true drying oils with turpentine. Some of the turpentine actually gets chemically bound up in the drying oil film, so not all of the turpentine you add to thin the drying oil based paint evaporates from it as it dries. It may be a mute point nowadays since no one sells drying oil based paints or varnishes anymore, but you can still buy Tung Oil and Boiled Linseed Oil, and if you thin them, use turpentine.