SO whats a poor filler?
What you in effect are doing is filing the pours with a clear binder/sealer and sometimes a filler...polyurethane, hide / hoof glues, varnish, shellac, white glue, epoxy, ca and sperm will in essence all do the same thing. I am not trying to gross you out here...these are all valid artists materials.
Typical fillers are. Sawdust, kaolin clay, talcum powder, chalk, ground glass, plaster and earth. Yup dirt. Or anything else that is fine enough to get into the pours, and binds into them .
Commercial pour filler makers add filler and various colourants to the binder of their particular persuasion. Some are clear.
So now... pause here and think about it...you got about a dozen different binders and powders that when mixed with colorant (you know, the stuff they squirt into that "can I have a gallon of this colour" at the uncle Wally or Colour Your World paint store stuff...that's colourant or if you want the proper name...its Japan Colours or Universal Tints) ...you gotta lotta choices.
So
binder + powder or filler = pore filler with colourant optional.
Simple. Now that you know...you can make your own.
Truth is...whatever you can shove into the pours that stay put and looks good is a valid pour filler...get it?
Personally I like fine drywall compound, colourant and some white glue or hide glue.
I hear that the long term side affect may be things like, poor intonation, warped neck, and broken strings oh ya and terrible headaches. But personally have never experienced any of them...well I have had the odd headaches when the kids were in their teens.
Blessings
the
Padma