I want to refinish very old barnboard floors. There are some small spaces that need filling . How do I fill in these spaces then sand the floor?

Hi,
I live in a barn with some areas having plank flooring,I didn't fill the spaces,I left them for appeal.
Refinishing the floor I used polyurethane.

I can explain more if you'd like.

Ken

If you start filling the cracks and variations you will lose the barn board effect. I would try to fight that temptation. Go with KBC