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?

 

called a profile sander...
multimaster is the top dog..
Bosch is next...
HF, PC, Rockwell and Ryobie are junk...

 

Peachy; welcome!
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 !!!

 

Thanks! This paneling has several grooves, so I might not be able to do what I need to do. It has been stained, and I want to stain it a lighter color.

 

bleach it... but experiment...

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...

for control use a grout sealer applicator...

 

Thanks, Stick486! I'll check into this! There are several grooves in the paneling about every 5-6".

 

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've attached a picture of the room I want to redo. The paneling is stained with a green pickling look. I want to lighten the stain. Thanks!

 

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.

 

Staining lighter is in theory at least, almost impossible. The ambient light goes through the coatings and is reflected back again through those same coatings. The amount of light you see, and it's apparent colour, has been reduced (dramatically!) from what went in.
The "lighter" colour will only be what's reflected back from the particles on the way in. You can't really stop the darker tones from reflecting back as well, hence what Stick said about bleaching the wood first!
If you have a MOHAWK FINISHES outlet close to you, i'd suggest you go and have a chat with them. They have a solution for just about any situation. Take a sample or a picture of the existing with you!
A lot of their products need to be sprayed, not brushed...
https://woodrepairproducts.com/wp-content/uploads/2016/05/PFS_Guide.pdf
https://www.mohawk-finishing.com/resources/ask-the-expert/
https://www.mohawk-finishing.com/where-to-buy/find-a-distributor/