flexing in plywood tanks - epoxy? rubber? or...?

[jjohnwm]

I've built a number of plywood tanks that were problem-free for many years (15+ in at least one case that I know of...) but I had a few leaks, and one or two of those were never properly diagnosed. All of those early tanks were done 20-30 years ago and were sealed with PalGuard epoxy paint, and ranged in size from 120 to 360 gallons. In the past few years I have returned to DIY tank construction and have put together a couple of 120-gallon plywood jobs, but these have been sealed with Pond Shield epoxy, a much more user-friendly product and much less terrifying to use in terms of off-gassing toxic fumes.

My newest project is a 360 gallon plywood tank, and the original plan was to use Pond Shield again...but now that I have read so many threads that allude to the cracking of epoxy caused by flexing in plywood tanks, I'm starting to question that idea. Maybe that was the cause of those few mysterious leaks in my old tanks?

I'm a firm believer in water changes; at least 50%, more often 75%, and they are done weekly. With this regular release and then re-application of pressure exerted on the tank, I'm wondering if the flexing/cracking issue is made worse. Is that big wooden box "breathing" and causing cracks with each inhale/exhale cycle? It's enough of a concern to me that I'm wondering if I should switch to Liquid Rubber or perhaps a flexible EPDM liner to address it...but then the installation of the front glass becomes way more involved.

This tank (like all my previous builds) will be a simple box of 3/4-inch plywood, braced on top and across the middle, with a glass window. I don't want to use Acrylic. I also don't use any fibreglass cloth reinforcement on inside surfaces or corners.

Any comments, suggestions or ideas would be appreciated.

 

[Tobiassorensen]

Not the need for being more intricate or complicated by using an epdm pondliner. Put in the liner and then you make an fram out of flat iron stainless steele and screw with silicone between steele and pondliner. Then you glue the glass against the stainless steele frame. Problem solved.

 

[fishdance]

You can water change by displacement so the tank doesn't have to flex. Adding 200 liters in will push 200 liters out. Just put in an overflow drain (drilled or over the side). It doesn't take much extra to automate the water change process once you have a drain.

Alternatively use a flexible membrane. Some are paint able.

 

[Guinness]

I had Line X spray the whole inside. Then 4 coats of sweetwater epoxy. This tank is only a few months old. I did one similar year's ago which held for a few years before I moved and had to cut it up.

 

[Guinness]

The sweetwater epoxy paint seems to stay mildly flexible but I don't think I'd use it on its own.

 

[jjohnwm]

Thanks to all for the comments!

I must say that the use of a liner does indeed sound like there are way more steps that each introduce another potential problem if not done perfectly. Just not comfortable with that idea.

The flow-in/flow-out displacement water change method is one I have used in the past, in my previous fishroom. I have considered it but rejected it here for two reasons. First, unless I turn off all water circulating devices during the change to minimize mixing of the new and old water, I would have to add much more water to actually achieve a given percentage of change, since the new water would be mixing in with the old and flowing out with it. My water source is a well, and I want to utilize it as efficiently as possible; don't want to pump 100 gallons up out of the ground, only to immediately dump 50 gallons back onto the ground. In the winter especially, directing the outflow from a major water change outside without causing unwanted ice patches is a bit of a production so I don't want to increase the work.

My second concern with a flow-through system is much simpler. If I set it up and use it for normal water changes it will indeed prevent any possible "breathing" or flexing caused by changing water pressure...but there are inevitable going to be times when I will want to drop the water level, or perhaps even empty the tank altogether, and so the concern about flexing goes unanswered.

Line-X? You mean truck bed liner? Now that really is thinking outside the box (literally...). I'm definintely going to check further into that.

Right now I am leaning towards a product called 670hs, which was mentioned in another thread on here. Can't remember which...I've lost track of all the tank build threads I've read...but it sounded good. Turns out that there is a dealer who carries it in stock right in the nearby city; up here in Canuckistan that never happens. A bit more research is required, but the stuff sounds perfect: low VOC, non-toxic, good adherence to bare wood, excellent coverage, tintable to any desired colour...and inexpensive! That's another thing that never happens up here. I can get a gallon kit, which sounds like way more than enough for this job, for about 70 bucks Canadian; that's around $3.75 US.... They are emailing me the MSDS and material specs now. I am hoping they can answer my question re: flexing.

 

[Guinness]

Yes, the truck bed liner. Silicone won't stick to it, but sweetwater epoxy does so you can't use it on it's own. Ask me how I know lol. I used herculiner on my first tank and it worked but I wanted even thicker and more durable on this one.