Polyurethane is not same as shellac.
All urethanes are plastics/polymers in a (usually) water based carrier.
Shellac is actually dried tree sap dissolved in alcohol.
"Shellac" will penetrate much more deeply and because it's a natural tree product, it will have a much different effect on the sound.
It serves to bind the wood together and make it less prone to splintering.
It will also brighten the sound which can be a good thing on luan as it doesn't have much high end resonance.
You can somewhat control the amount of "brightness" you add by varying the number of coats you apply and by sanding after it's dry.(shiny generally = brighter)
Keep in mind, you can only go so far with this. 20 coats won't make em sound like birch.
You want the "de-waxed" stuff and I recommend brushing it on with a natural bristle brush.
IMPORTANT: Be VERY careful what you do here.
You can destroy the vintage sound of a drum by doing this or any modification that involves the interior or bearing edge.
Be sure of your reasons for doing this - proceed at your own risk.
Good luck.