I've read where a lot of people said they never thin their paints to hand brush, so maybe in their area they must use a lot more paint then they do in my area.
See every bottle I ever bought has been the same, enamel, acrylic, it doesn't matter, if I don't thin it at least 40% the paint will cover the detail.
I've bought some Testors 1/4 ounce, and some MM, and some Tamiya. Just today I tried to paint my barbed wire with Tamiya F-56, and to get it to lay down I had to almost make it a wash, otherwise it would cover the detail.
Must be paint sets a lot longer around here, cause like I said, it is thick, and it isn't because of not mixing, I have an excellent paint shaker, and with 3 BB's in there nothing goes untouched.
Just for curiosity, I put 3 BB's in a bottle of thinner to watch how it does, and them BB's go everywhere, it is the same when I got paint on there, you can hear the BB's hitting the walls and lid, and it can go way faster then anybodys hand, plus last a lot longer.
So for the ones that say they don't need thinner to hand brush, the paint must be a lot thinner when it is newer, that could be the only answer I could think of for them being able to do that.

Straightedge

I always thin my paints when I paint by hand. I didn't for a long time and had horrible results painting details, especially headlights/tailights. After refering back to one of my Shep Paine books I began to thin to a 'ink like consistency' (as described in the book) when painting and I have better results. Although you can thin too much: last night I think I over thined some Testors Sky Blue enamel for the periscopes on my M981 and it ran a bit too much. So now I'll have to go back and touch up because the paint ran outside of the periscope glass paines.

But I like the 'ink' consistency because in a lot of cases its more like appliying on paint than making a physical brush stroke, which is difficult to do with small pieces sometimes.

I have begun the practice of almost always thinning my paint. I take is that its better to be thin than too thick. Besides, thick paint will show brush strokes much more. I believe FSM magazine just put out an article on painting with a brush, a few good tips in there, just read it at the news stand and save 5 bucks

Sean

This is basic painting skills that took me a long time to learn.
I have learned that dipping the brush in thinner first and applying a small smear of thinner to the area to paint and spraying thinner, first, through an airbrush before spraying paint will help drastically with the flow of paint.

I usually add a drop or two of thinner to the paint jar and shake it up to keep the paint from becoming viscous. As I use the paint, I also tend to dip my brush into thinner picking up just a small amount of the thinner. I also make sure that I do not use the same thinner for my non-metallic paints as I do for the metallics. Generally, the amount of thinner I use by dipping my brush into the thinner as I work , is just enough to keep the paint transfer to the work even and controlable. Some times I need the paint thinner, sometimes I don't. Dipping the brush in thinner also helps to keep the brush clean from coagulating globs of paint.

I would also suggest to keep the paint at an ink consistency and go from there to develope techniques that work best for you for each modeling situation.