I pulled up my carpet and sanded the flooring underneath with a drum sander with a 36 grit with the grain. After 3 passes I'm still seeing seams with the original poly and stain on them. It's very slight but I'm using a cooler tone stain and I'm worried those areas will stick out. Should I start from the beginning with a diagonal cut to even it out, or continue with the grain until they're the old finish is no longer there? And if I do have to start fresh with the diagonal cuts, how long do I continue with the 36 grit until I've over sanded?

Thanks for your input! I'm self taught and have only done one small room prior to taking on the whole house.

https://drive.google.com/file/d/1MH1nLj ... p=drivesdk

https://drive.google.com/file/d/1MF91W1 ... p=drivesdk

Can you isolate the dark boards with a diagonal criss-cross to get the areas sanded out? Just go 2-3 feet at a time, then go with the grain before going to the next paper. You probably need to only run 50 grit it cuts so fast.
It will be hard to see any low area after you are done. This works a lot better with an 8" vs. a 12" sander.