you can use any of the commercial wood filler products out there, but if you are filling a first plank hull in basswood use a light filler otherwise when you sand you will take away more basswood than filler and have to use more filler to fill those spots and you can see where this is going . for a double plank hull i use drywall mud as it sands smooth and is softer than the surrounding wood so you get a nice fade into the edges.for a single plank hull i assume you are talking about only small gaps or defects and you can mix sawdust from the same wood with a bit of glue. i put the glue directly in the spot/gap to be filled and sand around it pushing the sawdust into the glue until i get it right. let dry and sand smooth. hope this helps.