Poly is fantastic and wets out fabric much better than epoxy. With Poly you can also use mat (the glue binder in Fibeglass mat generally doesn't break down when using epoxy). Epoxy bonds better . . . and is less brittle. Both have their strengths . . . if you are wetting out large areas or doing transom job Poly is fine. If you are making parts in a mold . . . Poly is the ONLY way to go. patching holes under the waterline . . . I'd chose epoxy.
The only other time I'd use epoxy is when smell factor is an issue (working inside or in residential neighborhood with folks close by)
ISO poly was $125/pail when I bought it last year at composites one. Sometimes boat builders will sell you a few pails for $100'ish It's much cheaper when they buy 55gallon drums. Save epoxy for special occasions.
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