Ray Aldridge wrote:

When varnishing over
stained wood, how do you keep the varnish flat without risking sanding
through the stain? Clearly my brushing technique leaves much to be
desired, but when varnishing small boxes of stained poplar, I find it very
difficult to avoid all errors-- too much flow on edges, for example, which
leaves a line in the varnish. In the past, varnishing wood finished
without stain, I'd just sand the error flat with fine sandpaper. If I try
that with the stained boxes, it's hard to avoid sanding through the stain
at the edges.

Thin coats, just enough to wet the surface. Lay on each stroke, then
"tip" to lay the edges flat. Varnish against wet edges wherever
possible.