Peachy the easiest way would be to take a strip of wood that just fits the groove and glue some sandpaper to the end. Welcome to the forum by the way. Trying to get into the lips on either side is a lot trickier. Do you need to do that too?
Man, your question brought back old memories, and I don't mean good ones...
Back in the '50s there was a paneling product, basically hardboard, with triple grooves every foot or so (if I remember correctly).
Thee front face of the board was burnished to a hard smooth surface which didn't like paint
Trying to paint the grooves without leaving globs was a R.P.I.T.A !!!
Chlorine bleach, full strength, easily removes most dye-based stain but will not bleach raw wood white, nor will it remove pigment-based stain.
Two-part wood bleach takes the color out of most dark woods and blends maple heartwood color with its sapwood.
Apply A/B bleach safely. Wear long neoprene gloves, with ends cuffed to catch drips, a waterproof apron, and goggles. Brush carefully. A/B bleach is extremely caustic and will quickly burn your skin and eyes.
Oxalic acid dissolved in water removes black iron stains like magic from tannin-rich wood like oak.
30%+ peroxide is a good one to try also...
HEY STICK, I have a Dremel (piece of junk) that I bought in the 90's that still works. It has various rubber profiles that the sanding loop goes around and sands in odd places. Full disclosure: it has not been used in a commercial setting and probably would not hold up to daily use but it's reasonably inexpensive and for one off use it would work fine. Most of the time I follow Chucks method and just shape a wood block as needed and use adhesive backed sandpaper. That works best for us penny pinchers.
Never tried to stain the stuff but have filled the grooves and painted it.
I originally was thinking you meant T slots so disregard the part about lips in my original post. The part about just gluing sandpaper on a strip of wood still applies. I have a set of different sized dowels with 80 and 120 grits glued to them for rounded grooves, ogees, etc. Works well and costs virtually nothing.
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