Re: Transom. / paint or not?
The best way to see if your dealing with marine grade or not is to look at the edge of the board so you can see the seperate sheets laminated together. If there are holes and gaps its likly not marine grade and you should change it while the motors off.
If all of the the sheets are laminated nice and tight with no gaps or holes it is likly marine grade but you cant tell for sure with out knowing the glue used to laminate the wood.
What makes marine grade ply "marine grade" is one the glue used to laminate the sheets together has better water protection then normal ply and they take extra care to make sure that each sheet is free of defects such as holes, cracks and chips in it because they allow places for water to get in rot the wood and cause it to delam, so when you cut marine grade ply and look at the edge you will not see gaps or hole in the lamination as you would with normal ply.
And if you do paint it only use marine paint its designed to be less of a polutant then normal paint.