P120 is more of a finishing grade than one to start with. To get the paint up and marks out either scrape it or go over with P80 (or if really bad P60 then P80) sandpaper first. If the floor has already been lacquered then a stearated abrasive (generally white - the stearate is effectively a non-stick additive) is a better choice as it will load yp with dust less readily. Open coat abrasives perform better than closed coat abrasives on softwoods, which can glaze and clog easily if you don't clear them regularly. Then finish with P120 - I doubt you'll benefit from going to P180, but see what it's like after you've sanded to P120. Aluminium oxide (alox) sanding paper. Always clean up thoroughly when you change grits - a vacuum cleaner is ideal. Sand with the grain directionHow best to do this? Perhaps in the following order:
1) Sand the floor to remove as much of the paint and water stains as possible. Should I start with a rough sandpaper (120?) and then go over it again with a smoother sandpaper (180)? What type of sandpaper should I get?
2) Fill the gaps between floorboards with the sealant. (Wont it just fall down in the gaps? Perhaps theres a technique to this?)
3) 3 coats of varnish.