Having had the same issue, I found from a woodworker friend of mine to *not* sand the board. It seems the additional oil we (the foodies) put on the board isn't a good thing for the sanding. It clogs in both the sand paper and the wood pores.
He recommends "scraping" or planing the board to remove enough material. My dad made my board, walnut and hard maple, and I was extremely anxious to submit it to a planer. We knocked off between 1/64 and 1/32, reseasoned with block oil, and I've never been happier about it.
I've now quit putting those types of spices on my board. Onions, garlic, fresh herbies, and the like seem to come out with kosher salt and the other half of a lemon. Squeeze the juice, then apply the salt, then scrub with the open lemon face. Works for me.
As for the spices, I've graduated to a tub with sides for raw prep. After cooking, as much as I hate food-grade plastic, I use a large heavy board for cutting, mutilating, and generally "standing and snarfing" anything that's been smoked, grilled, or otherwise tortured in my metal chambers.
Now that I reserve the board for the less caustic materials, I'm on year 5 without a wood reduction session (through regular cleanings). My board still looks like it did from the planing and re-oiling.
Hope this helps ....