NOTE: This started as a brief addition at the bottom of the previous post, but then I ended up describing more details in an email to someone... then realized it's worth devoting a whole new post to wet sanding.
- 1200-1500 grit wet/dry sandpaper
- linseed or other painting medium oil
- small bowl for the oil
- clean, lint-free rag of synthetic material - a microfiber eyeglasses cleaning cloth works great! (cotton rags and paper towels have too much lint)
I used 1500-grit "wet/dry" sandpaper, moistened with a bit of linseed oil. I rubbed any area of the surface that was imperfect: too much medium, or a piece of embedded fiber or dust. It worked really well, I was amazed how the imperfections were healed by the process - most debris lifted right out and left the colors of the painting intact. Small unintentional drips or ridges sanded right off easily. I had to do a bit of touch-up painting in a few areas, but the process was a huge success.
I did the wetsanding on fairly-dry areas - maybe a week of drying. Dry to the touch, but you could probably still gouge or dent the surface if you tried. I was also willing to repaint whatever I messed up. (I wouldn't try it for the very first time on a masterpiece you thought was done and no longer have reference for, in case a little repainting is necessary.)
Also, it's not sanding like you sand a piece of furniture - I used a tiny folded square of sandpaper bent over one fingertip and pressed very VERY gently and rubbed in a very small area, only in areas that needed it.
Once I sanded I had a yucky layer of wet oil and loose lint, so I needed to find a lint-free way to wipe that off. I found what worked best was a microfiber eyeglass-cleaning cloth I bought at the local hardware store. I could wipe firmly enough to wipe off the wet oil and dust, without leaving additional lint dust like a paper towel or cotton rag would have.
One more tip: An accomplished painter I know just recommended using "shop cloths" as studio rags. They are extra heavy duty blue paper towels on a roll, I found them in the hardware store. They are amazingly lint-free. I had previously been using well-washed flour-sack dishcloths, and they seemed pretty lint-free but I now suspect they were adding to my dust problems - I seem to have a lot of very tiny white filaments flying around my studio. I'm going to try the shop cloth for a while for wiping brushes while I paint and see if that helps reduce the dust in the first place. (But I still would only use the microfiber eyeglass-cleaning cloth to actually wipe the surface of the painting.)