Q: I have pine floors from the 30’s. I had them professionally refinished two times in the past. There is no stain and the poly wears away quickly. I recently sanded one room myself and applied oil based poly. It came out darker where I sanded one area more than the rest due to scratches and wear in that area.
A: Perhaps the other finish used was a clear, non ambering water borne coating. That would explain the darker oil based. As a side note with oil based polyurethane on pine you should apply 3 coats. With a water borne, because they are lower solids finishes and hence don’t provide as thick a film you would need 4 coats for sure.
Follow-up: So should I sand that part and use water based poly?
A: If you have already used oil why don’t you stick with that? I’ve told you the characteristics of the two types of finish. Decide if you want the lighter look or a deeper colour and apply that to however much of the floor you want to refinish.