Myrtlewood/Walnut

I what would cause the finish to crack? coats too thick maybe? not enough drying time between coats? The finish is a water based polyurethane. over 2 coats of tru-oil to add some color to the wood. I have done this before on walnut with out issue. Sanding and recoating is not to difficult and I could go to a CA finish but am still curious what could have caused it. you can see it in the first picture that is the only place it happened.
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No plastic in this one @PaulFLUS but the myrtlewood is stabilized.

I was going to guess that the wood wasn't stable and expanded/contracted. Harrywally/Andrew told me that he sometimes had problems with certain finishes on wood cracking especially when shipping far away. He recommended when using CA finish to use one that could flex (I can't remember the exact brand off hand) to help avoid that problem. I would think that a poly would be flexable but IDK.

How long did you wait for the tru-oil to cure? If it’s just that edge it is more likely too much finish build up or not long enough cure time between coats. If the tru-oil hadn’t totally cured I would have expected to see some fish eye first then the cracking.

Rule of thumb generally speaking is never waterbase over oil base. Poly really isn’t what I would pick for a tool you expect to get wet, eventually the coating will fail. Maybe stick with the tru-oil and apply a few coats while spinning it on the lathe. That technique works really well with Waterlox.

Tru-oil is probably the best there is, for wood. Think about what a gun stock goes thru, in its life. Snow, rain, ect.