Quoted from MOSFET:Hardsuit: is there a specific polyurethane you can recommend? Do you have a recipe for your clear coat method?
https://www.homedepot.com/p/Rust-Oleum-Parks-Pro-Finisher-1-gal-Crystal-Clear-Satin-Water-Based-Polyurethane-for-Floors-258690/202521739
Strain the poly through a standard paper cone paint filter (4 pack is $1 at Home Depot). 3-4 layered coats using a quality bristle brush following the directions on the container for the timing between coats. Then a light scuff sand with a block using 150 grit sandpaper to remove nibs. Repeat as desired for how thick of a clear you want (personally believe more than 8 coats is excessive for this method). Once build to desired depth, again scuff sand with 220 on a block, then use a foam brush with freshly strained poly 1-2 times for final top coat. Let dry well, then buff as desired. The satin finish as linked above will buff out to a very high polish. I've successfully accelerated the time between coating layers by pointing a fan at the poly after each layer, and can usually start the next layer in under an hour. You do want to wait for at least 6-8 hours before the scuff sand stages, and 24-48 hours before buffing.
A disadvantage to this method/material is that you do not want to drip poly into cupped inserts to level them out, it tends to get a little milky when applied too thickly. Advantages are it's water based with (IMO) minimal odor and no real hazards compared to using auto clear, and that you can easily lightly scuff sand and add another layer on top if needed with minimal issues of blending edges if you get scratches you want to repair.
One 1 gallon container should me enough for 3-4 playfields, so approximate cost per playfield is in the $10 range for the polyurethane, plus another $10 for brushes, filter cones, etc.