I haven't screwed up....yet. Plywood type?

I joined the forum earlier today, treasure trove of info on here!! So much info, that I managed to miss the fact that you are not supposed to use pressure treated plywood on aluminum. (shoulda been tipped off by the "stainless steel hardware only" tag on the plywood =D> ) Haven't cut it or put it on yet, but before I take it back, can I just paint it or coat the side that will be in contact with the transom with something? Just load it up and take it back?

Take it back. You want exterior grade plywood.

Would that be the type that says "for interior use, and exterior use with proper treatment".....?

duckwhisperer said:

Would that be the type that says "for interior use, and exterior use with proper treatment".....?

Click to expand...

No.

I don't think no is the right answer. From what I have read on here you can use that or the exterior sheathing and both will work fine. It is vital that you treat it well with at least three coats of spar urethane...if you do that you should be fine with either type of plywood.

The simple answer is you want Exterior Grade Plywood. If your gluing 2 or more layers together you want a water proof glue like Tite bond III.

Here is a good reference on plywood types. https://www.buzzle.com/articles/plywood-grades-explained.html

OK....at our local Lowe's there are 3 types to choose from.......1) Labeled interior use only 2) labeled for interior or exterior (as long as it is properly treated) 3) pressure treated for exterior use only. Guess the second type is what I should use?

I'd check around at some other lumber stores. From my last project, there was a scrap piece and could read most of the MPA label. Exterior AC, group1 (not sure what that means). It is 19/32" and has 5 plys. I have had it installed in the boat for a few years with no issues. I plan to use that again, for next project. The original plywood from the '67 Starcraft was Ext AC.

There should be 4 types of plywood at your Lowe's.

CDX---Construction plywood with alot of voids and "football" fillers. The C is a grade for 1 side of the plywood. The D is a grade for the other side. Should be available in 3/8, 3 ply 15/32, 4 ply 15/32, 19/32, and 23/32.

BC Sanded---A little better grade. The B side will most likely be smooth because it is sanded with very minor imperfections if any. The C side will have a few imperfections. Should be available in 11/32, 15/32, 19/32, and 23/32.

AC---The A side will be perfect and sanded. The C side will have very few and small imperfections and it will be sanded as well. Should be available in 15/32 and 23/32.

Pressure Treated---As the name says, its pressure treated chemicals to make it resistant to moisture and insects. (stay away from this as it will react with the aluminum in your boat)

At the Lowe's I work at, the CDX, BC, and AC were all made with an "exterior glue". They all say for interior or weather protected use.

In my build, I used the 15/32 BC Sanded and coated it with several coats of the Helsman Spar Urethane.