How much sanding do I need to do to refinish a wood floor?

Question

I'm planning to sand a hardwood floor, restain it and apply a clear coat.

I tried using a random orbital palm sander and a 40 grit sand paper on an inconspicuous location. The sander does remove material, but I am not sure if enough material has been removed for applying a stain.

Is there a rule-of-thumb or a test that I can perform to ensure the quality of the sanding?

EDIT :

Thank you all for your comments. I rented a random orbital sander from a hardware store and these are the results :

What I did :

1. Used a rented random orbital floor and used 36,60,80 & 100 grit sand papers

2. Used a floor edger with 36(big mistake) , 60 , 80 , 100 grit

3. The floor edger ate up a lot of wood. I had to rent a square buffer floor sander and used 60grit and 100 grit papers.

The buffer helped feather out the edge with the rest of the floors. I'm hoping this does not become a big issue when I apply the finish. I'm applying a dark finish.

Answer 1

You'll probably want to rent a proper floor sander. A small random-orbit will take ages and you'll probably burn through most of its useful life.

The sander should come with a variety of paper grits. As with any woodworking project, start with the heaviest and transition to the finest. The final should be somewhere in the 100-120 grit range.

More information

How far you need to go depends partly on the stain color--is it markedly different from the current color? Is it dark? Dark stain is more likely to compound if it's not completely removed. You'll see the old stain through the new as dark streaks or patches.

If you go the route of renting a sander, the work will be easy enough that you can get through the old stain without much effort, and you'll be confident of achieving a good result.

Answer 2

Hopefully you are using the palm sander for a small area. If not, you'll have your work cut-out for you. If the floor has gouged or scratched marks you should try to sand them out with a low grit (36-40) paper. If they are deeper than a 1/16 inch it may be best to sand a wide enough area to make them less obvious.

To remove the old stain start with 40 grit paper and work your way through each ascending grit finishing with 150-180 grit. When you are sanding with the 80-100 grit you should see that the wood is starting to brighten (depending on the wood species) and most of the old stain is being removed. So that when you switch to 120- 150 grit most of the sanding will be for smoothing the rough wood and removing small areas of light stain. The final pass with the 180 is for removing any swirl marks from the previous sanding grit.

If you intend to use a water based stain you may want to save the final sand with 180 grit until you damp mop the floor. Use a lightly damp rag to pick-up any dust. The moisture also raises thee grain of the wood which would occur when you applied the water based stain. Let dry and sand.