The finish on that stool is probably shellac, which is dissolved by alcohol and degraded by ammonia.

You can over coat with polyurethane(water based would be the better type) but the finish on the stool has to be dry and hard. Or completely removed.

Depending non the age of the shellac, regular denatured or isopropyl alcohol will either dissolve the top layer(less old) or make a gooey mess(older in decades).

You might try just sanding the seat with 220 grit sandpaper by hand and then rough up the other parts and apply the poly.

"Depending non the age of the shellac, regular denatured or isopropyl alcohol will either dissolve the top layer(less old) or make a gooey mess(older in decades). "

isoporpyl contains water (unless you pay for the anhydrouse stuff, $$).

Denatured alcohol is what you want to remove shellac.

Cover with paper towels, soak with alcohol, cover with decent thickens plastic (NOT the super thin 'drop cloth' plastic).

Wait about 10-20 minutes, then dampen the towels with more alcohol and use them to wipe up the mess.

ANY age of shellac will come off this way.

Thank you both for your comments.

Will using Denatured alcohol turn the rough & rustic nature of my bar stool into something more smooth & finished?

Not what I would want when all I want ot do is seal it.

pbx2 - It will not change the texture of the wood.

If the shellac you remove is an amber shellac, it may lighten it a bit, but the poly will darken it a bit.

The alternative is to live with the rough and rustic sticky mess.