How To Sand A Wood Floor

How to Sand a wooden floorI often get asked about floor sanding – the machines, the sandpaper grits, correct sand paper sequence and methodology.  To that end, I’m going to answer some of the most common questions asked by first timers or DIY sanders. For the purposes of covering the basics, I’m assuming that you’re using the three standard sanding machines: the big machine, the edger and the buffer.

How to Sand a floorBear in mind that hire machine are not dust-free. You will need to empty out all bookcases, take picture and curtains off the walls and open all doors and windows to get as much air cross flow as possible. Also hire machines have low power motors so undulations will exist as the machine starts, stops and encounters knots in the floor. You will need to clean down the walls, shelves and all nooks and crannies, not just in the room sanded, but possible throughout the house. The finish will also contain “trash” (imperfections) as the dust settled over many hours on the wet varnish.


1)What grade of sand paper do I use to start and finish?

The grade of sand paper is actually called the grit and refers to the particle size on the belt of sand paper.  Your goal in a sanding sequence is to use progressively finer abrasives to flatten the floor and smooth out the wood to get it ready to accept new finish.  All the while taking off as little wood as possible so that the life of the floor is maximised. If you skip more than one grit in the sanding sequence, you end up with the first cut leaving deep scratches in the wood, which the second cut will be unable to take out.  These visible scratch lines are called peak and valley profiles in the finish.Sanding a Floor process

This peak-and-valley profile will leave a rough-looking floor and, if stained, cause uneven staining. Another result of skipping grits is early finish wear in heavy traffic areas. The reason is that the peaks will not have as much finish on them as the valleys, so the finish on the peaks will wear off faster.


2) How do I know what grit to start with?

No two floors are exactly the same—similar, maybe, but not the same. Always inspect, repair and clean (sweep and vacuum) each floor prior to sanding. Yes, this even applies to new wood floor installations, as well as cleaning between grits on the big machine. (In many cases you can pick up an abrasive “hitchhiker” from the previous grit and cause wild scratches.)

For new floors, one of the best ways to choose a sequence is to select the finest grade you want to finish with and work back to the coarser grades, skipping one grade between each sanding.

For example, if you want your finest grade to be 100, the sequence you would follow is 40-60-100 (skipping only grades 50 and 80). If you want to finish the job with grade 80, use the sequence 36-50-80 (skipping only grades 40 and 60).  Use only as heavy a paper as it takes to do the job. If you don’t put the deep scratch in, you don’t have to take the deep scratch out!

Sand paper is graded for wood flooring as follows: 36g – 40g – 50g – 60g – 80g – 100g – 120g.  I’d suggest going with 40g – 60g – 100g as the sanding sequence as it only skips one grit of sand paper at a time.  Also 36g can be a nightmare to remove the scratch line so better off to do a double pass with 40g rather than use 36g.


3) How do I know what grit to end with?

A floor sander is much like an artist with a blank canvas, they have control of how the final result will look. Know what is achievable — floor varnishes have a far higher solid content than furniture varnishes so the finish on a baby grand piano is not achievable.  Furniture is also sprayed in a specialist spray booth or in a UV cure machine in ideal conditions.  A wood floor is varnished in site conditions and the varnish is applied with a roller.  Don’t get me wrong, a properly sanded and sealed wood floor will be smooth and look great just don’t expect it to be like furniture.

Rarely have I seen floors sanded finer than 120g on the big machine. If you are sanding a typical No. 1 common American White Oak strip floor.  Finishing with clear varnish, you have many options; usually either 80 or 100 grit will be the final cut made with the big machine. Keep in mind if staining then finish as recommended by the stain manufacturer.  Instructions are usually on the tin (normally 120g).

Also remember that too fine of a final cut with the sanding machines can close the grain of the wood, causing poor adhesion of the finish coat and making the stain appear lighter. There is no good alternative to experience, but you do not have to reinvent the wheel:  call me on 087-6268100 and I’ll sand and seal for you J with my dust-free machines.


4) Should I use the big machine or edger first?

This depends on the expertise of the person sanding. When you become more experienced and can have smooth cut transitions at the wall lines, then it becomes a preference. For beginners, I strongly recommend using the big machine before the edger. Beginners with the big machine tend not to have a smooth transition cut at the wall line, often leaving a drum or “beauty” mark in the floor, as well as uneven distances between the walls and the sanded area.

Doing the edging afterwards means you know exactly how far out you have to edge, since you can see the sanded area, and if drum marks are made at this point in the sanding sequence, then edging can remove them. An experienced contractor on the big machine cuts the floor as close to the wall lines as possible, leaving less for the person edging (unless you have it in for the person edging—then stop about two feet away from the wall!).

Making smooth drum cuts with the big machine takes practice, and every big machine needs to be adjusted properly. The motion with the drum should be like a plane with smooth take-offs and landings, always in motion.  Always slowly engage the drum to the floor while moving forward or backwards and gain lift the drum off the floor slowly while moving.


5) Should I go right to left or left to right with the big machine?

The type of big machine determines the direction. With the older drum-type big machine, you start at the right side of the area to be sanded and move to the left. Belt machine manufacturers recommend the opposite.

The reason is because of the carriage wheel location on the big machines. On the split drum machine, the carriage wheels are directly behind the drum, while on the belt machines, the left carriage wheel is left of the drum. As the floor is cut, you want the carriage wheels to follow in an area that has already been sanded so as not to transfer over-wood of boards to the drum, causing sanding irregularities.  Look up the manufacture and see what they recommend or ask the machine supplier.  If the machine supplier doesn’t know, walk away and find a better supplier who does know.


6) Why should I go at a slight angle with my first pass on the big machine?

Doing this serves a couple of important functions. By going at an angle of 7 to 15 degrees to the direction of the floor boards, you cut the floor flat the fastest. Why? By going at an angle you are cutting slightly across the grain. The cross-grain cut sands the wood floor quicker and also takes out any slight rolling or waves of the floor. Once you create these cross-grain scratches with your first cut, you need to cut straight with the grain so the scratches go with the grain for the second or final cut (depending on the grit sequence).  It also massively helps to remove any over-wood or under-wood at the end of each board.


7) Where do I start edging a wood floor?

Before you start edging, you need to be aware of where the cutting point is for that edger so you can control the cut pattern.  To find this out, just slightly touch the sanding pad off the floor.  The result will show you where the edger is sanding.  It’s usually about the size of a €2 coin.

Assuming the edger is set up and working properly, you start on the left and work to the right around the perimeter of an area. The rotary cut of the edger is the most aggressive cut of all the floor sanding machines; knowing how to keep the scratch pattern with the grain as much as possible minimizes edger swirl marks. A typical first cut with an edger is 60 or 80 grit, then completed with 100 grit. To soften the cut of the abrasive, there are edger pads that are used under the bolt with an abrasive disc for the final edging. By using edger pads, the abrasive disc cuts less aggressively and also leaves a finer scratch.


8) When is the best time to trowel-fill the floor?

Trowel-filling the floor prior to the last cut with the big machine and final edging provides a sanding guide for the beginning floor sander on the last abrasive cut. The trowel-fill will show low spots and any other marks or areas that need sanding attention with the next machine, the buffer.  See my article on floor filling.

Always let filler dry thoroughly before sanding. If you sand and the filler is not dry, you will load up the abrasive and use more abrasive than necessary. Prior to trowel-filling is a great time to make sure that all nails (called “shiners”) are set. If you sand the nail heads, you will leave sanding streaks/lines in the floor from where the abrasive did not cut the floor flat (the abrasive will be missing where it hit the shiner).

In many cases, the buffer will not completely flatten the floor where there is a shiner, but you will not detect this phenomenon until the last coat of finish is applied. (This is called Murphy’s Law!)


9) When do I use the buffer and what grit on a wood floor?

Use the buffer just prior to applying the varnish.  The purpose of the buffer is to “blend” the final rotary scratch left from the big machine (in the field of the floor) and the final circular scratch left from the edger (around the perimeter). To reduce initial scratching, when starting with new sand paper, always start in a dark area of the room or other inconspicuous location.

Use the same grit on the buffer as you finished on the big machine, this will usually be 80g or 100g.  Between applying coats of varnish, use the buffer again, this time on 120g or 150g to smooth out the wood fibres caught in the varnish.


10) What else do I need to know?

Keep the job site clean at all times – no dogs, dirty shoes, people walking through, etc.  Keep a cross flow of air at all times to try to minimise the dust.  Wear a dust mask and ear protectors as the machine are very noisy.  Empty the dust bags when half full to ensure the machine is picking up as much dust as possible.  Take your time and don’t get frustrated as rushing the job will lead to a poor finish.  Have a good shower afterwards as you will be covered in dust.  Best of luck with it!!!

Alternative, if the hassle, dust and poor finish with hire machines is not your cup of tea, then just give me a call 087-6268100.


On the Safe Side

I would be remiss in any article about sanding if I didn’t mention safety. Keep the following in mind:

  • Sanding dust is highly flammable. To minimize the risk, never operate the sanding machine when the bag is more than half full. Also, never leave the sanding machine with dust in the bag or any bag of dust in your vehicle or house.
  • Use eye, ear and respiratory protection devices as required by OSHA. In particular, the sawdust from exotic woods can cause adverse reactions in some people. Don’t forget to use approved respiratory protection when emptying dust bags or dust collection systems.
  • Make sure you have safe electrical hook-ups
  • Have all machine guards in place.
  • Keep electrical cords away from the machines’ moving parts. Also, keep cords out from underfoot and off your shoulders.
  • Unplug all machines when you are repairing or adjusting them, or when changing abrasives. 
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