Some time ago I had a project that left me with two plywood containers that I had to scrap. One was coated with a water based 2 part epoxy paint for concrete floors and the other was coated with Westsystem solvent free 2 part epoxy. I decided testing how strong the coatings were and started banging on them with different objects. To my astonishment they were not strong at all! The coatings cracked like eggshell. Plain float glass was much stronger. So I decided to change my view on how plywood tanks should be coated. I have documented my tests in a non scientific way here.
A small piece of plastic, and an old screwdriver.
Picture shows the corners that were used to make the holes.
Two thick layers of Westsystem 2 part epoxy. Clear.
Three layers 2 part water based epoxy paint, bought at “Biltema” in Norway. Gray.
Standard float glass, 4mm.
I tried gradually weaker tapping with both plastic and screwdriver. Both epoxy coatings consequently cracked, even with relatively weak tapping. Every crack exposed the wood beneath. The glass only got tiny marks even with the hardest tapping with the screwdriver. Any harder than that and I was afraid of cracking the 4mm glass plate to pieces. The plastic made no marks on the glass.
The paint offered little resistance to even weak tapping. Notice the largest hole. The loose pieces were removed from it to show how much wood that is really exposed.
The Westsystem epoxy was a little stronger, but hardly enough to mention. Here holes from the plastic piece.
And here the screwdriver.
It's a bit hard to see, but the glass piece lies on top here. There are small marks on it. They can be seen only because of the glass dust around them. When that was wiped away they could only be noticed by careful inspection. The marks were made by hard tapping with the screwdriver.
The reason it is so simple to crack these coatings is probably that the wood itself is very soft. So you get this relatively brittle and thin layer on top of a soft body. There is nothing under the coating to stop the point pressure attack and the coating layer is on its own to stop it. It yields, and since it is too brittle to bend, it cracks. A thicker coating, say 4 layers of 2 part Westsystem epoxy, would help a lot. But only fibreglass gives us a real solution.
Every one of the dents in the coatings is enough to ruin a plywood tank by itself. Once there is a hole the wood sucks up water and makes the plywood swell and eventually rot. So it’s game over for the whole tank. Think about how easy such a point pressure could occur in a large tank full of rocks. So my conclusion is that one should use fibreglass reinforcement to get a stronger coating in large plywood tanks.
Since posting this article I have seen one case where a tank has actually been ruined because there was no reinforcement in the epoxy. A man who regularly posts in the DIY section of Cichlid-forum, username "lomax", explained that the epoxy layer in one of his tanks was ruptured during the installation of the glass sheet. The heavy weight of the glass combined with sharp edges was a fatal combination. This would not have happened if the epoxy was reinforced with fibreglass.