The problem happens when the lighter wood is porous, in which case the sanding dust from the darker wood gets into the pores of the lighter wood and thus “contaminates” it.
I don’t use poplar, so I can’t say whether it is porous enough to cause problems. Usually, people use maple as a light contrasting wood with walnut, and maple is not porous so it does not create a problem.
If you are concerned, there are a few options.
One is to cut out the poplar and replace it with maple. :)
Two is to put a coat of shellac on the poplar while it is “uncontaminated”, and that should provide pretty good protection against the walnut sanding dust.
Three—- and this is what I would do—is sand the top until you are happy with it, then use a card scraper to scrape the poplar and surrounding area until it’s as fresh and clean as you want it. Then apply the finish.
-- Kelby
