Ken:

Interesting point. My experience with oak is negligible. With figured
walnut, using the sealer first results in the figure showing nicely
IMHO although I haven't done a really good experiment to compare
sealer/no sealer. I'll try to remember to put that on my list of stuff
to try. But for the last couple of years, lighter color woods have
sold better for me and I haven't worked walnut very often, even though
it's fairl;y easy to find here

Kip