Want to varnish mdf skirting

Unfortunately, varnish is not going to cover any remaining white paint on your skirting. Also the varnish will soak in to MDF, which is porous, so this will make the problem worse. There are only two things you can do in my opinion; 1. Completely remove ALL of the remaining white paint from the skirting. This is the hardest option as, like you said, the sander cannot reach all areas, but is the only way to get the finish you desire. Take some sandpaper (120 grade, followed by 180 grade to smooth it out before varnish) and try to remove the last of it by hand. This is really the only way you will achieve the varnished look with no white coming through as varnish is clear by nature and not good at covering other colours. 2. Paint it with something else. I know this is not what you want to hear but its the easier option without a doubt. People generally do not varnish MDF woodwork, its not real timber so has no grain and wont look amazing even if its brand new. Re-prime it with a water-based primer, then solvent-based undercoat and whatever top coat you wish after that. Hope this helps

Unfortunately, varnish is not going to cover any remaining white paint on your skirting. Also the varnish will soak in to MDF, which is porous, so this will make the problem worse.