A good deal of the board sold in this country as marine ply is ordinary ply with a stamp on it. There are three ways to check and all of them take a minute.
Marine plywood costs roughly twice what commercial plywood costs, which creates an obvious temptation, and in seventeen years of buying board I have been offered ordinary ply as marine ply more times than I can count. If you make bathroom or kitchen furniture, this is worth a minute of your time, because the failure lands on you rather than on whoever sold the sheet.
There are two real differences. The glue, which in marine ply is a phenolic resin that does not soften when it is wet, and the core, which in marine ply has no voids and uses a denser timber. Everything you can check follows from those.
First, look at the cut edge, properly, in good light. A marine sheet shows an even stack of plies with no gaps between them. Commercial ply has visible voids, small pockets where a core strip did not quite meet the next one, and once you have seen the difference on two sheets side by side you will never fail to spot it again. Ask a supplier to cut a corner off. A supplier who will not is telling you something.
Second, the glue line. Phenolic glue is dark, almost black, and you can see it as a thin dark line between plies on a clean cut. The urea glue in commercial ply is pale and often invisible. This is the quickest check and it is nearly conclusive.
Third, the boil test, which is the only one that actually proves it and takes twenty minutes. Cut a piece about four inches square, boil it in a pan of water for ten minutes, let it cool, then try to lever the plies apart with a chisel. Marine ply will not come apart. Commercial ply separates with almost no effort, and a sheet that fails this test would have done exactly the same thing in a bathroom over two years, just more slowly.
Weight is a weaker signal and still useful. A marine sheet in 18mm is noticeably heavier than a commercial one because there is more timber and less air in it. If you handle board daily you will feel it.
And a word about stamps. Every sheet arriving in this country has something printed on it, and the printing is the cheapest part of the sheet. I have seen the same stamp on two sheets that were plainly different products. Do not buy on the stamp.
What we do here is simple. We will cut a corner off any sheet you are considering, we will tell you which of our sheets is which, and we will talk you out of marine ply where you do not need it, because a wardrobe carcass in a dry bedroom does not need a boil proof glue and you should not pay for one.
