Q: Would you apply waterbased pore filler directly onto wood or give it a wash coat of something like shellac first?
A: With most waterbased fillers, you can do either, but there are reasons to go one direction or the other. If you are not staining the wood prior to filling, and don’t mind the stain in the filler coloring not just the pores but also the background wood, you can apply filler directly to the raw wood. You can, incidentally, stain filled wood afterwards with most pigmented stains.
On the other hand, if you want the filler color only in the pores, or if you have dyed the raw wood and don’t want to risk cutting through the color while sanding filler, or contaminating it with a contrasting filler color during application, then it’s best to seal the wood with a coat of Zinsser SealCoat prior to pore filling.