Hand Brushing Acrylic Paint

Once upon a time, Tamiya acrylics could be hand-brushed straight out of the bottle with silky smooth results.

However, since the early 90's, Tamiya's acrylics have a tendency to skin over very quickly, with the result that when your brush travels over a previously coated area, it causes an effect which we call "roll-up", where it drags up the previous layer of semi-dried paint in lumps.

This can be overcome by adding Tamiya thinner to your paint. Tamiya thinner is said to contain an acrylic retarder which extends the drying time of the paint, reducing the rapid skinning effect and giving it more time to "level". A similar result may be achieved using Isopropyl alcohol plus a drop or two if acrylic retarder (which can be found at art supply stores).

I have found that Tamiya paints can be thinned rather heavily and still get good coverage.

Assuming that the acrylic layer is fully cured (at least 48 hours) you shouldn't have any problems overcoating with enamels.

Essentially, what Phil said. However, I'd like to add a few modifications:

Don't thin or add retarder to the whole bottle, as this could destabilize the paint. Decant a small amount, add thinner and retarder, and discard after you're done with the brush.

Always dissolve retarder in thinner before adding to paint. 

Be careful with retarder and small amounts of paint. Some Tamiya colors don't react well to high concentrations of retarder in small amounts of paint. What I do is keep a bottle of 90% isopropyl with retarder already dissolved in it. This is what I use when hand brushing, and also it's convenient when airbrushing.

For small areas, what Phil said about enamel over acrylic is correct. However, larger areas may still give trouble under some circumstances. If the acrylic was not well bonded to the substrate (because the substrate was not clean enough) shrinkage of the enamel as it cures may pull up the underlying paint. Alternatively, applying an enamel over a larger area of acrylic gloss may result in crawling, as the enamel may not bond well to the glossy acrylic surface. Light sanding the area with very fine grit (2000 or so) will prevent this problem.

Some enamels do very well when undercoated with Future. Some do not.