Re: marine plywood... necessity or not?
I kinda agree on the PT thing. I mean, it's mostly for reassurance of the owner. If the wood gets wet it won't rot, but it will swell up hard enough to tear thin fiberglass, and the weight will still be there. Who wants a soaked boat even if it's not going to rot? Just take it out and fix it.
On the ply question, marine vs. exterior grade is kind of a religious issue for many. But, here is some info on the subject.
First, use marine grade if you're engaging in a project where the designer specifies it... marine is stronger and stiffer than exterior because of the number of plies, so if a designer calculated in that strength you'll want to make sure you use the materials he expected.
Marine is easy to work with... it's usually very smooth sanded, very flat, holds dimensions well, and looks good. Certified for marine use, it may be important to use this for insurance purposes.
Marine ply is stronger than exterior. This is because of two things. It has a larger number of plies, usually twice as many as exterior, which improves its strength tremendously. Second, it has no voids... all the plies are continuous with no air gaps inside the wood. This means you get uniform strength across the piece and no little reservoirs for water to collect.
Marine ply is NOT treated against rot. Common myth. Sometimes the wood it's made from is high quality tropical hardwood (so marine ply can be less eco-friendly than exterior) which has some rot resistance, but there's nothing special about the marine name as far as rot goes. You can get marine PT plywood, which is both marine and rot treated, but the same comment as above applies for it... who wants to have a non rotting boat that's soaked with water?
In reality most production boats are not made with marine ply. They're built using exterior of some sort, and more and more new boats have no wood at all. Using exterior on your repair or modification is a pretty safe thing to do, especially if you follow the rules for encapsulating it in glass or resin.
I use Arauco ply (from a factory in Chile) and plyfloor underlayment (baltic birch ply) for stuff right now. I can get both at Menards, both have almost no voids, the Arauco is AC (not officially X yet, but it has the same waterproofing) and usually has both sides almost AA and is renewable, being made from fast growing plantation trees instead of wild grown hardwood. Both are $20-$30 a sheet. More expensive than CDX, but much nicer to work with.
Erik
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This post is part of Plyco’s Guide to Marine Plywood and highlights the important differences between Marine and Exterior Grade Plywood.
It’s one of our most commonly asked questions regarding Marine Plywood. Without clear information, it can be extremely difficult to choose a side in the Marine Plywood vs. Exterior Plywood battle. On the surface, they seem extremely similar, but knowing the key differences between Marine Plywood and Exterior Plywood can save you from a lot of headaches further down the line.
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The Similarities Between Marine Plywood and Exterior Plywood
Before we get into the details of what separates these Plywood cousins, it’s important to acknowledge their similarities. The most obvious similarity is that both Marine Plywood and Exterior Plywood are designed to be used externally. Both products utilise an A-bond glue, allowing them to stand strong in the face of all the unique challenges that come with being used in the unpredictable outdoors. You’ll also see similar face grades between Plyco’s Hardwood Exterior Plywood and some of the cheaper Marine Ply options (like Pacific Maple Marine Plywood). From here however, the two products begin to diverge from one another and begin to display how unique they are.
Marvellous Marine Plywood
While the A-bond glue used in Exterior Plywood makes it similar to a Marine Plywood, there is so much more going on under the hood that the comparisons actually start to look a little bit silly. Firstly, to be classified as a Marine Plywood, the product needs to meet strict standards. Regardless of whether it’s British Standard or Australian Standard, a Marine Ply needs to go that extra mile to actually receive the “marine” title officially.
For greater detail, you can read about this in our “What is Marine Plywood?” blog post, but the summary version is that: (i) there must be absolutely no core-gaps, (ii) the veneers used must be of the highest quality, and (iii) the glue needs to be waterproof. Without just one of these factors present a Plywood cannot be considered marine grade.
Another large difference is aesthetic quality. Particularly in our Gaboon Marine Plywood and Hoop Pine AA Marine Plywood options, the aesthetic quality is significantly higher than what you would find in an Exterior Grade Plywood. This difference is why we often see customers using Exterior Plywood for something like a trailer, or various storage applications where appearance isn't important. In comparison, Plyco’s Marine Plywood will be often used for its stunning looks to create magnificent boats, as it strikes a balance between the strength and water-adaptive qualities and the visual qualities.
Exceptional Exterior Plywood
A big difference that our Exterior Plywood offers is that it comes in three different sheet sizes, allowing for greater flexibility when building. Because of this, our Hardwood Exterior Plywood is a good choice when you need an exterior product, but don’t necessarily need something Marine Grade. As mentioned, a Marine Ply has to be knot free in order to get its official classification, but with Exterior Plywood you can find knots of up to 1 and a half inches present! Similarly, you can also find core gaps present, although they do have to be under an inch wide or the Ply will lose its exterior grading.
Exterior Plywood and Marine Plywood are closely related, however we hope this post has been able to help you distinguish these two outdoor Ply boards from one another. If the Marine Plywood vs Exterior Plywood debate has been settled and you know exactly what you’re looking for, head on over to Plyco’s online Plywood store where you can shop our entire product range. Alternatively, if you’re still looking for more information you should take a read of our blog post “The Key Things to Know About Marine Plywood vs MDF”.
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