Caporegime
Stay away from the ready mixed fillers, theyre a nightmare to sand down. Pack the deeper holes out with wet kitchen roll, poke it in with a screwdriver. Then mix some powder filler, (tetrion or pollycells pollyfilla are my personal choices at work). Fill the holes slightly proud to allow for shrinkage and sanding. On the deeper holes were the surface skim coat has came away, build the layers of filler up gradually until you get them level.