New floor treatments won't adhere or look nice if the old varnish remains. Strip a floor using one of three methods: sanding (time-consuming), chemical stripping (messy) and heat stripping. The method you chose depends on your comfort level and expertise. Safe and non-toxic strippers on the market can minimise danger when using them. Most of these are some sort of orange or citrus based product.
Vacuum the floor to pick up and debris that might rip the sandpaper. Load the belt sander with the heavy grit paper. Put on the dust mask and turn on the sander, lowering it slowly to the floor. Immediately move it smoothly up and down in the direction of the wood grain.
Sand over each area until the varnish is gone. You will have to change paper very frequently as it will clog up with the old varnish and become less useful. Use long even strokes and don't let the sander sit in one place as it will sand out a dip in the floor.
Vacuum the dust up continuously to keep the room comfortable to work in and keep debris from the sandpaper. When the varnish is removed, change to medium grit paper and begin smoothing the surface in preparation for new floor treatment.
Vacuum the floors. Open window and assure adequate ventilation. Put on the gloves and apply the stripper in the direction of the grain. Let it stand the amount of time that is recommended on the package. The varnish should bubble up and away from the wood.
Use the putty knife to loosen the bubbles of varnish. Scrape in the direction of the wood grain removing curls of varnish and stripper. Dispose of the old varnish and reapply stripper to any areas where the varnish is still adhering.
Wipe down the floor with white spirit to clean any vestiges of stripper. The white spirit will evaporate quickly leaving no residue. Vacuum the floor again and begin refinishing process.
Plug in the heat gun and turn it on. Hold it at an angle a couple of inches from the floor and move it up and down in a small area. Varnish will bubble up and loosen. Do not hold the gun too close or the varnish will smoke and loose wood will burn.
Use the wood scraper to remove each small area of loosened varnish. Scrape downward in a smooth motion. Don't press hard or gouge the wood. Use the sharpener often on the wood scraper blade. A sharp tool works better and is less likely to slip and make a mark.
Repeat the heat gun process in small patches until all the varnish is removed. Load a belt sander with medium grit sand paper and sand down any rough patches or left over varnish. Vacuum the floor and prepare for refinishing.