Why does everyone recommend finishing floor sanding with a high (e.g., 120) grit sandpaper?
What would the downfalls of leaving the last sanding at 60 grit be?
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Finishing with a low grit sandpaper like 40 grit or 60 grit will leave too many scratches on the floor, making it rough. When you apply stain and finish, these scratches will actually be visible. Use the 40 grit and 60 grit for surfacing, then finish with 100 grit or 120 grit. Especially with stain, which will pick up on scratches, you don't want to skip the final sanding. It should be less visible if only finishing with a polyurethane, but is still an important part of doing a job properly.
I had three rooms to redo and decided to experiment. One room I used only 30 grit on, another I used 30 grit and then 60 grit, the last I used 30, 60, and 120. I used a satin oil based polyurethane to finish the floors.
The result? No noticeable difference between the final finish of the three rooms.
However - the amount of air bubbles that occurred during the finishing process was strongly related to how many levels of grit was used. The room which only received 30 grit had a ridiculous amount of air bubbles. The room with 30 and 60 had less, and the 120 grit room barely had any. Considering that the first room I applied polyurethane to was the 30,60 grit room, the air bubbles were most likely not strongly related to a learning curve. Still - these air bubbles can very effectively be removed by sanding between coats.
In the end, the notable "amateurisms" about the floors are a few divots due to edge sanding and a few other minor details related to the finishing process (a lap mark and a couple 'slick' spots).