Ok, I am going ot be starting the sanding of my floor this weekend in prep for staining and ifnishing. I have an large orbital sander reserved, and plan on starting with 60 or 80 grit (not much needs to be sanded, as it is pretty clear and level already), then going up to 100, applying wood filler, the finishing with 120gr.

However, the renter tells me they only go up to 100 grit paper. Is it ok, to finish Douglas fir flooring with only 100 grit? Isn't that a touch rough?

If not, then I will be stuck using my personal DeWalt orbital palm sander with 120 gr hook and loop discs for the entire floor.

I finish fir with 100 grit on my drum sander. The final paper will be dulled just a little with the machine running, lightly touch the drum paper with a used edger paper to knock off the sharp points of the grit.
Fir will not need to be grain filled, so only spot fill nail holes or cracks with wood patch, red oak color. Butt-joints may need to be top-nailed.
If you don't have a sub-floor I recommend blocking underneath with a strip of one-half inch plywood 2 inches wide glued between joists if the flooring can be accessed from below to help the floor hold up over time, before the sanding takes place. This will take a load off the top part of the groove which is more important if the floor has been sanded before. Any gaps between the planks will hold the wood patch better after blocking, also.
Depending on the finish that is on the floor already, the larger orbital sander may hurry the removal and smoothing of any thing on the surface. Then use your small random after you scrape the corners. You can get right up to the base board.
If this is the first sanding you can get by with a small random to remove the mill marks. 80 grit should be OK for a natural finish "poly" with a random orbit sander.
In our 1906 house the floors have never been sanded with a machine. The finishers only smoothed the edges, so the floor is not perfectly flat, to prevent splinters. Then it was sealed and varnished.

Actually, this is a new install. I cleared all wood prior to install, so no knots, but left the character, so it has some heart, and also various pieces of rift to spice it up. I have a gallon of Minwax Early American we like the look of a lot, so that's the plan. Alos a coupke gallons of Varathane High Traffic oil based finish.

There are various boards with edging not perfectly level, so I would prefer to use the big orbital to level these areas. Wasn't planning on grain filling, just wanted to fill cracks, nails, and a few pitch pockets.

I actually bought the Parks Pro White Oak. It seems to have a good match, but you say Red Oak is better?

Ok, so I'll sand up to 100, then fill, and just go over again with 100? Sounds good to me if it works.

Also, I have noticed quite a bit of uneven and blotchy stain on my sample pieces. I can't help but think I really should throw on a coat of Minwax Pre-stain conditioner to help prevent this.

I have never used a 12X 18 orbital that the rentals provide. Do they have 100 grit screens to use after the 100 grit sandpaper? I have never used pre-conditioner, but I have read that the conditioner will make the stain color lighter.
Fir accepts stain according to how the grain direction is oriented to the surface. When the grain is parallel it is one color and when the grain raises or lowers the stain absorbs more. I can use #2 steel wool to polish off the darker areas by rubbing hard where it has absorbed more and leaves less blotchiness.
Red oak color has the warm color that fir will develop as it ages. For a natural finish it's best. With a stain many fillers will take the stain color to match the wood next to it. See if you can scoop out any pitch that is not solid before you fill the
pocket. I fill any holes before the 100 grit paper cause there can be ghost patches around the fill spots that 100 may miss on the final sanding paper.
With a stain you may need to go over the whole floor with the 120 grit paper on a smaller random orbit. Try a closet or an area out of the way before you do the whole floor. If you have extra wood try a stain sample piece separate from the floor with both the pre-treat and with the 100 grit sanding sequence before you even try the floor staining to get the results you want. When the stain is still wet during application this is the color that you will get with the first coat of finish.
You may need to hand sand with 80 grit with the grain to blend in the corners where no machine will reach and you use the scraper, which can tear the grain.

Yep, I've been experimenting with samples, and I finally got exactly what I was looking for.

I took four random 12" scrap pieces and just glued them together to get an idea of how well the pre-stain conditioner would work on various grains. All I can say, is that it's miraculous. Yes, the overall is just a tiny bit lighter, but far more even. Very little blotching, like before. The sample I finished with conditioner and 120gr orbital looks fantastic.

In fact, the Early American stain went from super dark blotchy areas to a nice even golden brown over the entire surface, and all the character of the pitch and heart wood grain is beautifully accented, and not ugly like before. I'm very impressed.

I also experimented with trying a 1 lbs cut of shellac as a conditioner, but it didn't do nearly as good a job as the Minwax. If you ever want to stain a soft wood like fir or pine, I highly recommend this stuff. So far, it's got my vote.

So, this is the plan. I got the sander. I will spend the day tomorrow starting with 80 grit, then 100 grit. Then, I will fill all cracks and pits etc. Once dry, I will go over the whole thing with 100 grit again. Then, I'll be done with the sander.

Since the rental sander does not have 100 grit screens.... Monday, I will spend the day with my little orbital to finish it up with 120 grit, and prep for the wood conditioner and stain.

Once the stain is dry, I will have to wait until next weekend to Urethane. I may just put some heavy furniture blankets on the floor in the hallway so we can walk on it for a couple days with old socks until I can Urethane. Not much I can do about it. Hopefully some clean furniture blankets will protect the stain/surface for a few days without causing any issues.

Dear God, this is messy!!!

ha

I feel the pain. I just started sanding a manufactured laminate to get the finish off and then polyU. My test area looks great but no mater what I do there is dust all over the house!!!!