It’s generally better to use a dedicated paste wood filler (pore filler) to fill pores than the finish itself, or sanding sealer, because finishes continue shrinking. This will cause the pores to noticeably open up a little after a few weeks or months.
But you can use the finish for filling, especially if it’s water-based because water-based finishes sand fairly easily and don’t shrink as much as varnish and lacquer. Apply several coats, sand them back using a flat block to back your sandpaper, and continue doing this until the pores are filled.
Varnish and lacquer are more difficult to sand because they tend to gum up the sandpaper. Here’s a trick to make the sanding easier and make it less likely that you’ll sand through.
First apply a coat of the finish, varnish or lacquer, whichever you are using. Follow with several coats of varnish sanding sealer or lacquer sanding sealer. Sanding sealers sand much easier than the finish itself, which is the reason they are sometimes used for the first coat.
Then sand the sanding sealer, as shown in the accompanying picture, until you feel the resistance of the coat of finish at the bottom, which is the varnish or lacquer. Stop when you feel this, clean off the sanding dust and decide if you need to apply and sand more coats of sanding sealer to fill the pores level.
Finally, apply several coats of varnish or lacquer to finish up.