After using some cheap brushes, I finally ordered a set of W&N 7 brushes today (# 0,1,2,3). I'll use them for oil washes and detail painting (I use Vellejo acrylics). Can I use the same brush for both oil & acrylic?

Sure, Chris.

Just make sure that they're clean and dry before you switch mediums.

You might get better results painting the acrylics with Series 7 liner brushes instead of rounds or brights. The bristles are much longer (the points are still small and sharp) and the acrylic paint will flow better from them than it does from rounds.

The liner brushes work very well for pin washes, too, for the same reason.

I also use Vallejo acrylics for all my brush painting (with a few other acrylic brands for special colors), and one of the keys is to thin the paint properly and use multiple coats. If you try to achieve single-coat coverage, it's very easy to get the paint built up too thick - introducing brush marks and hiding details. The liner brushes work very nicely with paint thinned to the consistency that I like to use.

Another point when painting with acrylics is to keep the brushes clean as you paint. When you notice the paint will not flow from the bristles, give the brush a quick swish and swirl in some water, wipe off the excess and return to painting.

Acrylics work by flowing off the bristles which simply direct the point where the paint will leave the brush. Once on the surface, you don't move acrylic paint around too much and never try to "brush it out." Simply allow it to dry in place. However oil paints are pushed around by the bristles and smoothed out ("brushed out") on the surface. So, longer bristles that "flow" are good for acrylics and shorter bristles that can "push" paint are good for oils.

In any event, the W&N Series 7 brushes will work well with any medium, and if you clean them properly, they'll last a very long time.

HTH,