That thickness sounds about right. I just redid my 20 and it was about that. If I was gonna do it again I would do it from the inside. It may seem like more work but after doing it from the outside I doubt it. Also doing it from the inside means it does not have to look perfect like the outside. Doing it from the inside is probably stronger too. I'd strongly consider doing it from the inside. Lots of info on here just search transom and you'll get all the info you could ever want
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