Hi guys,

I have refinished an old oak floor, and did a pretty good job, but this is different so am coming for help.

I have a newly installed birch floor. Raw cut, right from the mill. It is quite smooth as is, but some of the boards are touch thicker than some of the others - a dozen or so. I would say they are maybe 1/16" at most thicker on one edge.

I am in a tough situation because the rental company has a drum sander, but only 36, 80, and 100 rolls.... I am hesitant to go from 36 to 80.

Would a random orbital sander with the proper grit do anything for the unevenness of the floor? If so, what grits would you suggest?

This is for a cabin, so perfection is not the goal. The boards already seem quite smooth, i am tempted to just give a it a quick sand with the orbital at 80grit and put the finish down, but if i have the orbital rental and can get ride of the ridges than i might as well...

And one thing: I live in a very isolated area, and I have VERY few options. Closest home depot/lows/rona is 300 miles away. The rental company i am using is shipping me the sander on a truck. My point: options are limited, so please keep that in mind.

Section question related to the one above (sorry i couldnt edit).

do I absolutely have to sand the floor for the finish to take (oil based varathane)? i plan to do 3 coats, and think that might level out the unevenness enough for it to not bother me.

You certainly may sand and finish without using a drum sander. I have a house that has never been sanded with a drum sander. The reason for sanding is to remove the over wood. A belt sander, 3X 21 with a 60 grit belt will remove thicker edges. You may sand diagonally to even out a thick board. Go over the board with the grain to remove the diagonal scratches. Then use a 80 grit paper to smooth the 60 grit.
A random orbit with 100 will smooth the whole floor for finish to remove mill marks from the molder.
Use some knee pads and get right down to the floor. It will be a lot of work, but you only need to do it once.
In the very old days, the floor was prepared by scraping the edges of boards, then hand sanded with a pole and weighted block with sand paper wrapped around it. My fir floors are over a hundred years old and recently were coated with 2 coats of water-based finish, (25 years ago).
I would recommend a colored poly-coat with colored poly to give the old time varnish look instead of staining, Then protect with 2 more coats of "clear" poly.

I took another look at the floor, and the overwood was very minimal actually. This mill is known for high quality flooring.

A local expert recommended, from experience, that a 40 followed by 80 on the orbital will do a nice job to the specific wood from the specific mill in question. However, 40 is a bit difficult to get where i am, unless i can wait and get it in the mail (which I cannot).

The rental company only has 36 and 80. The lady running the desk said she's "never heard complaints about that combo in 20 years." ...

The local expert as well as reading online advises against it...

I think what i might do is 36, and then see what 80 does in a small section. If it does not working, then try some 60, and then go to 80.

I found some 60 at a home depot and have a friend pick it up...

Finish will stick to the freshly milled flooring. Sanding will remove the mill marks from the molder that the knives leave as the wood is getting shaped. You may not notice them unless you have good light. The finish will penetrate a little deeper if the surface is not polished by the milling.