Yes, once you "think" you are done sanding, and apply a black primer, you will see ANY imperfections in the sanding and prep job. Then it will be back to sanding.
Several coats of primer (8 or 10) will fill in minor imperfections with wet sanding (600, 800, and finally 1000 grit).
You can start your sanding with 100 grit but you will be removing tons of wood. I would not go this route. Since you will be filling in the dents and gouges with filler, there is no need to sand away hugh amounts of wood. I would start with 220 grit, buy good sandpaper which lasts about 10 times longer than the cheep stuff.
Use the following steps: 1. Strip the existing finish and a little rough sanding with 150 might be ok. 2. Fill the holes with filler and let dry. 3. Sand with 220 to get a smooth finish. 4. Second treatment of filler for missed spots. 5. Sand with 220 and then 400 grit. 6. Primer. 7. Fix the imperfection. 8. primer and wet sand.
Talk to an automotive body shop man or professional wood restorer for tips in preparing a surface for painting.
PS: If it were me, I would stain it real dark and not cover up all of the wood's beauty. This way the imperfections will not be as noticable and you would have a lot less preperation work before painting. Talk to a body man about painting anything high-gloss black and they will tell it IS tough to get out all of the imperfections. Of couse if you go the stain route, you will not be doing the black primer steps.
My parents have a baby-grand stained very dark from the factory. it has lots of nicks in it but still look great and with stained wood, a few nicks do not detract from the beauty.