Quoted from Tuborg

The marine stuff yellows with time.   I believe it'll yellow more than the poly..

Quoted from mrniceguy

...and I thought that Varathane supposedly yellows less than Poly.  Am I...gasp....mistaken ?

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Other Urethane Alternatives to Varathane.
Although Varathane is not the ultimate answer to playfield clear coating, it does have some advantages. It won't yellow, it's inexpensive, and it's easy to apply.

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Another question is about spray can polyurethanes seen at the Home Depot type stores. Avoid these products as they often contain linseed oil (which causes the finish to yellow with time). Here's a description of these consumer spray polyurethanes, taken from a manufacturer's web site: