I wanted to refinish my Rimu (New Zealand native timber) floors but I was doing it in the evenings and weekends and I didn't want to rent a floor sander for weeks at a time (OK, I'm cheap). At the hire shop, they had big belt floor sanders but also a big circular floor sander. I thought the circular floor sander looked strikingly similar to a floor polisher that cleaners use, so I started trawling the second hand market. I got a floor polisher for $70, which was a bit more than the daily rental rate of the real thing.
So on to the conversion....
Step 1: Disassembly
The floor polisher only came with one brush and in hindsight, the easiest conversion would have been to somehow shave off the bristles and screw a round plate into the wooden back. At the time, I thought I might need the brush for something but I haven't yet.
The brush base was bolted to a flange plate (like a quick release attachment - Photo 3) and there was a corresponding flange on the machine (Photo 4). Photo 5 shows the side view of them just separated (when they are locked together there isn't room for the camera lens). After I got them apart, I realized that I just needed to attach this flange plate to a sanding plate, attach some sandpaper and I could start sanding!
Step 2: Build the Sanding Plate and Attach the Sandpaper
The top plate of the brush (darker hardboard disk on top) was bolted to the flange so I decided to re-use it and cut some plywood disks to form the sanding plate.
At this point, I went around Christchurch looking for a supplier of sanding disks and I suggest you do the same. Different places, different sizes, different suppliers etc... I found lots of 250mm (10") diameter disks and also 305mm (12") disks. I wanted to use the bigger disks, but, the polisher was set up for 250mm brushes. I ended up cutting two 250mm diameter disks out of 19mm (3/4") ply to make sure it cleared the polisher's shroud, then a 305mm diameter disk out of thinner ply to attach the sandpaper to.
I cut the disks by what I'm calling the "builder's compass method". Nail a nail into what will be the centre of the disk, attach a string to the nail and a pencil at the desired diameter. Draw a circle with the pencil, and cut it out with a jigsaw.
The sandpaper I found didn't have any adhesive or velcro on it so I just stuck it to the sanding plate with spray-on contact glue. IMPORTANT: Find a contact glue that says it is "Temporary" or "Removable". The trick to making it removable is to spray the contact glue on one side only (I only sprayed on the sandpaper, not the plate), let it go tacky (1-2 minutes depending on temperature humidity etc...), then slap it on the sanding plate.
The original brush had a foam dampener and acted like a shock absorber. If you plan on doing lots of floors, its a good idea! Without it, the sander sends a lot of vibration through your arms and is very sensitive to small hand movements. When I built this, I had no idea whether it was going to work, and certainly didn't think I was going to use it for as many renovation projects as I have (and loaned to friends for their projects).
Step 3: Sand! Sand! Sand!
You've got your floor sander and now you are ready to sand. The general rule of sanding is move from rough to smooth in incremental steps. The first step removes the previous finish along with lumps and bumps. Subsequent steps only remove the scratch marks from the previous steps. By the time you get to 120 grit, you'll have a really hard time doing anything but making the surface "a bit smoother".
I tend to follow the following progression:
25 grit: If you have some old linoleum or glue or paint to remove, start here.
40 grit: If you have old varnish or polyurethane, start here.
80 grit: If you have new timber or chipboard or a very smooth surface, start here
100 grit: If you have cork tiles, start here.
120 grit: Finish here.
While your starting point might be different, you pretty much have to move through each step until you get to 120 grit. If you skip a step, you run the risk of leaving scratches from previous steps.
Also, don't forget your eyes, nose, and lungs. Wear safety equipment. Rimu dust is toxic so I went a bit over the top with fan forced ventilator (found it second hand) and it was much better than dust masks I used early on.
Step 4: A Few Additional Thoughts
These aren't in any logical order, just a few ideas for those who have actually build the machine and want to start sanding:
1) Start by getting a selection of each type of grit that is available. You never know how they will work with your machine. Once you find what works best, you can get more of your favourite grits.
2) After the first half hour, an 80 grit acts like a 100 grit and on down. As a cheap person, I've been known to save the old 80s and re-use them as 100s later....
3) Find a cleaning supply store and get an extra drive belt or two as backups. If you don't use them, it doesn't matter, but it seems that things tend to break when you don't have a backup...
4) When you get the machine, practice with it on your garage floor before converting it to a sander. When you see someone using one, it seems like they are effortlessly swinging it from side to side, but the sandpaper has a lot more resistance than the brushes or cleaning pads.
In fact, they are not swinging at all, they are moving the handle up and down. If the sander is rotating clockwise, lifting the handle puts more force on the sandpaper that is furthest from you which is moving to the right. This makes the sander want to move to the left. Check with Newton to find out why. Conversely, if you push the handle down, more pressure is put on the near side of the sandpaper which is moving to the left, so the sander wants to go right.
5) Stay with the courser grit until the floor looks smooth and uniform. Don't worry about the scratches, they'll come out in the finer grits. Also, the finer grits tend to clog very quickly with anything that isn't wood, so don't be tempted to jump to the finer grits prematurely.
6) Once you have a 305mm power sander, you start to use it for all kinds of things you hadn't thought about. When it's not used for sanding floors, I put it upside down between two saw horses in the corner of the workshop. Last week I used it to resurface a grooved disk rotor from one of the kids' cars. You can see the rotor-shaped metalic colouring in the middle of the photo. Last month I used it to slowly grind down an old Minolta lens mount so I could convert it to a Canon EOS mount.
Step 5: Finish
There are hundreds of floor options from organic wax to toxic 2-part gymnasium finishes. If you set out criteria at the beginning, you can shorten the search. I wanted low VOCs, flat or low sheen finish, roller application, and easy to re-coat in the future. We ended up with a tung oil sealer that needs to be re-done every 5 years or so, but we've been able to do patches of the hardest worn areas without re-doing the whole floor.
If you choose a hard wearing finish like a polyurethane or a 2-part finish, it will last much longer, but at that time you'll have to sand it all off and start again. If you had a hard time getting the old organic varnish off the floor, imagine how much work it will be to sand the hard wearing stuff!
You can see that the first coat of the tung oil sealer (photo 1) dries quite blotchy, but the second coat (photo 2) is much more even. We stopped at the third coat (photo 3 & photo in Introduction) because each coat after that would get glossier.
We lightly sanded between coats. I vice-gripped a 1/3 sheet sander (with 120 grit) to a pole and ran it around the room like a push broom. It only sands off anything that is sticking up above the surface (speck of dust, broom bristle, pebble, splinter, etc...). If you don't, they get bigger as they get covered by additional coats.
Hope this was helpful and good luck with your floor refinishing!