I have some purpleheart that I'm finishing. I read so many conflicting posts about what to use that I just went with what I know best (doesn't mean I know it well - please keep that in mind). I'd read people who seemed to know what they were talking about say not to use water-based finishes, not to use oil-based finishes - both gave logical sounding reasons why doing so would mess with the natural pigments - one said not to use BLO, another said use BLO. I decided I would just let the chips fall where they may regarding the color retention and try to get a nice finish otherwise.
I sanded to 320, let it sit and get nice deep color, then mixed BLO, wipe on varnish (Formby's Tung Oil Finish - apparently contains a lot more Formby and Finish than Tung Oil!) and Naphtha, say a few big glugs of BLO, a few smaller glugs of Formby's and a bit less Naphtha. I wet sanded with 320, 400, will do another coat or two with 600, maybe higher if I'm bored. It went on nicely and brought out wonderful depth. I'm at the stage of wiping off the specks that rise to the surface after the first coat.
I wasn't working in good light (lesson learned) and walked out into the sun to admire my results. Oops. I hadn't properly filled the grain. I've read that this approach fills small grain features with muck made from the finish and wood dust. There was plenty of that around on the surface as I sanded but I think I wiped it all off. Note that most of the surface is very very smooth but there are what I think are medullary rays - running mostly along the grain - and these are the areas that need sealing. I've worked mostly with hard maple and not encountered this, and with oak, which I filled with CA and ran a card scraper over to get a glass-smooth finish, but not good color.
Any suggestions? How do you wipe down the surface after wet sanding and not wipe off all the muck? I assume any filler product would need to match the color of the wood.