Wash Technique and Thinner Question

When doing washes over enamels, it's best to use the already-mentioned barrier coat of clear to protect the underlying paint.  If that's not an option, then I would use some Windson & Newton water base oils, thinned with distilled water and a tiny touch of dish soap to help break the surface tension.  HTH!

Jon,

There are two things you can try:

1. Try doing your base painting in acrylics.  In Shep's book, he actually talks about base painting in acrylics.  If your thinners aren't too strong, they won't lift the underlying paint when you brush the thinners on.  I've tried base painting in enamels and then spraying on a protective barrier of acrylic clear.  The problem with this technique is, if you miss any areas spraying on the protective barrier (like corners), you'll have painting flaking off again from the thinners.

2. If you base paint in enamels, you could try an acrylic wash.  Acrylics are a little harder and  less forgiving when used as a wash.  The trick is not to flood the model with the wash but only apply the wash in specific areas (like panel lines). 

I actually base paint in acrylics and do my washes in acrylics too.

Jesse 

You need a barrier coat of clear first, acrylic over enamel or enamel over  acrylics.