Never heard of using powdered pumice to fill wood pores....I have used it to polish the surface during the finishing process.
Read:
http://en.wikipedia.org/w/index.php?title=French_polish&action=edit§ion=2 The process is lengthy and very repetitive. There are also many similar variations in schedule and technique. What is described here is one such schedule. The finish is obtained through a specific combination of different rubbing motions (generally circles and figure-eights), waiting for considerable time, building up layers of polish and then spiriting off any streaks left in the surface.
The pad is first used to put a thinned coat of shellac on, then thicker coats with small amounts of superfine pumice, a crushed volcanic glass. The pumice acts both as a fine abrasive and to fill the pores of open-grain woods. Each coat must be fully dry before the next application, to avoid lifting out the softened finish.
It's seriously arcane, time-consuming, labor-intensive and gives the worker forearms like Popeye. It also yields an amazing result. The pumice is key when working with woods like mahogany, where more open grain comes along regularly.
