I am resurfacing hardwood floors and I have finished the upstairs. I used stain and then 3 coats of Minwax floor poly from Menards. I slid a small gunsafe which was less then 5 lbs on the floor and it scratched. I am looking for a better poly and options of filling the gaps in between the boards.
I’m guessing your poly from Minwax was a water-based one? While all polys will scratch, oil-based poly dries softer and is more flexible and resistant to scratches than water-based.
Water-based poly dries harder and is better for dents and knocks. Minwax does an oil-based poly that should do the trick: https://geni.us/JKoDbR1.
On sealing between the floorboards, the only real way to seal gaps is to use thin pieces of wood glued into the gap and then sanded back to level.
You can buy strips, but you can also make your own on a table saw. It’s a long and arduous process, but it works well. People try to use sealant, but given the movement in floorboards, it’s only a matter of time before it breaks up.