I'm going to sand and varnish the floorboards in our spare room, I have prepared the floor and made sure all nails and banged down etc, however I wanted to fill the gaps between the boards so I got some wood filler.
The stuff keeps falling between the cracks, even the small cracks, it's driving me insane, i would leave it but I'm terrified of spiders so want to block any way into the room!!!!
I looked at the project also, it only gives the one option which as you say may not be suitable for all sizes of gaps.
I have just completed sanding of a wooden floor in an old house, the floorboards have various gap sizes between 1 and 6mm. Ive had a variety of recommendations;
String soaked in hard oil - I tried that but due to the gaps it was unworkable.
Sawdust and glue Ive used before for small fillings in wooden furniture etc but dont think it would be practical for this, need huge amount of sawdust and a bit messy.
The current option is, stuff strips of newspaper into the gaps to 3-5mm below the top, mask the edges and then complete with a filler. I have a wood filler but its not recommended for floorboard gaps so I will be looking for an as yet elusive flexible filler.