Your floor needs to be sanded in stages with progressively finer abrasives for the best finish. Starting with grit 40, then grit 60 and finally grit 100 is tipically a good procedure for most wood floors.
Start with coarse sandpaper of 36 to 40 grit, progress to a medium 60-grit paper and finish with a finer 100 grit. Do not skip the progression from coarse grades to finer grades. Replace the abrasive belt after sanding about 250 square feet for most effective results.
The rule to remember is that, no matter what grit you choose as your starting point, you must sand, in order, with every grit that is finer than your starting point. So, if you start with 16-grit, you cannot jump to 36-grit; you must go 16 > 24 > 36 > 60 > 80 on both machines.
But the sad truth is that sanding is not like renting a Rug Doctor—it will take at least four passes, each with a progressively finer level of sandpaper, to truly refinish most old floors. And the hardest thing you're going to do on the project is to determine the perfect grit starting pass for your floor.
As a general rule, if you started with 80-grit paper, skip to 120 grit paper, or if 100 then to 150. Finally, sand with 220 grit sandpaper. Note that it is essential to know the type of wood you are finishing before starting to sand as this has a direct relationship to the grit used for final sanding.
Sanding with the grain is the golden rule of woodworking, which means moving the sandpaper in the same direction as the wood grain. It helps to minimize visible scratches and results in a smoother, more professional-looking finish.
To minimize swirl marks, always sand with the grain and use fine-grit sandpaper for the final sanding stages. Keep the sander moving and avoid pressing too hard on one spot. If swirl marks do appear, lightly sand the area with fine-grit sandpaper (220 or higher) by hand, moving with the grain.
Using the wrong sandpaper grit can result in either an insufficiently sanded floor or a floor that's been sanded too aggressively. What Not to Do: Do not start with fine-grit sandpaper, and do not use excessively coarse sandpaper unless necessary.
One of the most common mistakes is over-sanding, which can lead to an uneven, distorted floor surface. This typically occurs when you focus too much on a specific area or try to refinish a stubborn knot, bump, or defect in the wood.
For the middle of the floor you can use a pole sander with a 120-grit screen, or a floor buffer with a pad and a 150-grit screen. If you have more that 200sqft to screen, we recommend using a buffer. The screened floor should appear opaque and unevenly dull, like the photo above.
You want to start with a more abrasive grit and then use successively finer grits. The first grit is meant to take off any finish, the next is to smooth out the pattern of the first grit, and the final grit is meant to smooth out any remaining sander marks and provide the finishing touches.
With enough time and energy spent sanding, you can still end up with the same result. However, because we skipped so many grits, it is going to take much more time and effort to lower the 80-grit peaks and establish the 180-grit scratch pattern that we need for a high-level finish.
That is: what grit to start with, what grit to end with, and what grits to use in between. This process is sometimes referred to as a sanding schedule or sanding sequence or more simply, going through the grits.
Your floor needs to be sanded in stages with progressively finer abrasives for the best finish. Starting with grit 40, then grit 60 and finally grit 100 is tipically a good procedure for most wood floors. The goal with the first coarse abrasive is to remove the old surface coating and to flatten the surface.
Use a random-orbit sander for flat floors. It's easier to handle, and the random sanding pattern won't leave directional scratches on the floor. Floors that are cupped and wavy require a drum sander. It's best to call a professional for that.
Isolated sanding marks can be addressed by repairing/resanding individual areas. Resanding the affected areas may require the first cut with the big machine to be on a steep angle, followed up with straight cuts. Use of a hard plate, multi-disc or planetary sander also will help alleviate some sanding marks.
So, if you're sanding an average sized room where the floorboards are in average to poor condition we would say you'll need: 3-4 sheets and 3-4 discs P40 coarse grit. 3 sheets and 3 discs P80 medium grit. 1-2 sheets and 2 discs P120 fine grit.
Depending on a floor's thickness and the experience of the flooring professional, a floor can be sanded around 4-6 times in its entire life. Hardwood floors can be refinished every 7-10 years, making the 4-6 refinishes more than enough.
When you have deep scratches and dents on the floor which have to be removed, the drum sander is the best choice. For light sanding when you do not have any major and visible problem area, the orbital sander is a good choice.
Lambswool Applicators
The method recommended for applying oil-based polyurethane to hardwood floors is with the use of a lambswool applicator on a wooden block. One word of caution with lambswool applicators: The fibers of these applicators have a tendency to get stuck in wet finish.
Common Wood Sanding Grit Progression
Usually, 60 grit is the lowest grit experienced makers will use on wood and that's only if the piece has gouges, splinters, loose fibers, or dips. A popular grit progression for wood is 80 grit followed by 120 grit, 150 grit, 180 grit, and 220 grit.
Sawdust or grit caught under the paper can leave unwanted scratches. To know when you are done sanding, look at the wood in a low-angle reflected light. Or wet the wood then look at it from different angles. Before you apply your finish, raise the grain by wiping the surface with water.
Avoid sanding too fast, or too slowly.
However, like most things where quality and attention to detail are important, moving the sander at a controlled and even pace across the surface will lead to the best results.