Don't sand them , you will alter the fit...
Use Klean-Strip or Citrus strip both are water clean up
take old flat pan or make a tin foil tray ...paint the grips and allow to work
Use old credit card of or a gift card as scraper you will want to do 2 or 3 applications and cardings
wash off the excess , I then use acetone it will get any remaining poly varnish and will evaporate water quickly
You may want to do this twice also....
Your grips should be down to bare wood, allow to dry and use 0000 Steel wool , you are ready to finish
You can use Tung oil, Danish oil if the grips are not colored the way you like
Personally I use just Skidmores Wood restorer and Johnsons Finish wax