Um... no. MDO is far weaker than marine grade, unless you're comparing thick MDO to thin marine. MDO is a poor choice for any boat work, because it's designed to have a smooth face for sign painting. It's paper faced. So when you glass anything to it, you're glassing to a piece of paper glued to a piece of wood. This is actually worse than glassing to plywood with a thin veneer on the outer layer... you want to glass over a layer of wood that's not going to pull off easily.
There are several dissertations on the net about marine ply, and even some sellers don't know what they're talking about, so I encourage you to look up more info from the manufacturers and see what's what, don't just go by what you hear on this board.
Marine ply is made from several different types of wood. For example, two types readily available are okoume and meranti, which are actually species of wood if I'm not mistaken.
Marine ply is:
- Made with waterproof (usually phenolic) glue
- Made with no voids
- Made with evenly thick plies (no paper thin surface veneers)
- Sanded both sides
- Not repaired (no footballs)
- Sometimes Lloyd's certified for boat use; This may not matter, or may be critical in obtaining insurance or selling a boat.
Marine ply is NOT:
- Rot proof (or even rot resistant, that depends on wood type)
- cheap ($$$)
- As easy to obtain as exterior ply or treated ply
Treated wood is fine for boat use IF you dry it first - it's usually got far too much water in it to use as-is. But using treated wood is less important than making sure whatever you use gets sealed with fiberglass and poly resin or epoxy. You don't want whatever wood you use to soak up water, period. Seal it and seal any holes you make in it.
Short answer to your question, look elsewhere and buy marine if you want to be conservative.. anyone who tries to sell you treated instead of marine has no idea what you really need. All the boat books and some marine architects will say to repair your boat using marine ply and epoxy.. both materials are more forgiving of weakness or poor technique than lesser ply and poly resin. But that's the safe answer for people who may get sued for giving bad advice.
If you want to do the repairs or work cheaply, use a good quality exterior grade, with as few voids as possible. Exterior grade differs from marine ply in just a few ways:
- It's weaker (fewer plies and more voids)
- It's cheaper
- It's usually a less expensive wood, sometimes softer
But a repair done properly using a good exterior ply will last longer than you. A repair poorly done using marine ply will rot. A repair poorly done using treated ply won't rot, but will swell, cracking fiberglass and making your boat extra heavy.
Look around and do some research... this is probably the most common question asked on boat repair boards, and a quick google search on the differences between marine ply and other plywood will set you straight.
Final note: If you have a production boat from most makers, the original ply used probably was exterior or similar. Not treated, and not marine. However, if you know what was used by the maker, it's always safe to duplicate materials. I also know one marine architect who designs boats for home building, and specifies marine ply because he calculates the extra strength into his designs. The designer is always right
Erik
