Plywood alternative method of sealing

[Jon_DK]

Hi all
I plan to build a large tank. I have considered different methods, and the "plywood-way" seems to be the easiest one. However as many others, i am not sure how to seal it. My interpretion of threads and articles is that people uses the epoxy/glasfiber or liquid rubber method. But both has drawbacks. Looking just at the finished result i like the epoxy/glasfiber method the most, but this seems to me also as the most complex way to do the construction. I have nok experience with epoxy or glasfiber. Epoxy is expensive, and bying the right one also is not easy in denmark. Another big drawback to me is the toxicity.
One drawback obviously with epoxy is that it is not very elastic. A was told that a board of plywood with length of 240cm can shrink/expand easily +/-1.5mm due to (room) humidity changes. This causes severe stress in all plywood junctions and the epoxy would easily crack (at least if not armed with fiber). Still by arming it with fiber i feel that the construction is not fully controlled. In other words i would also like the liquid rubber method which easily adapts to the geometry changes of the plywood box. Liquid rubber however are not riggid to handle a sharp rock punching against it. It also has this brownish color, and i guess the scraping of algeas will be tricky.

So is there any other methods. I have thougt of an alternative way (see attached jpg).
Using glass/silicone the traditional way, but using the plywood as a full support for the glass. This way much thinner glass can be used (this is what my logic tells me). In all corners where one piece of glass meets another an adequate volume of silicone is applied to seal and to absorb not only the movements due to water pressing against the whole structure, but also the movement caused by the plywood box shrinking or expanding.
If this will works one could have surfaces that are easily cleaned using an algea scraper. The glass can be painted any color on the backside (towards the plywood).

I have thought of similar methods using sheets of acrylic, but refused that idea because i don't know an elastic sealant that sticks to acrylic (that are also UV-resistant and healthy for living organism). Acrylic or other plastic material however would be nice because it tolerates irragularities (not as fragile and unpredictable as glass).

What do you think ?
Do you know threads/articles on similar thoughts ?
Am i missing something ?

Thanks in advance.

Best regards, Jon.

 

[J. H.]

So, I'm a big fan of this HDPE Panel you can get at Menards. It is about 55$ for 4' x 8' x 1/4" sheet. Unlike a epoxy or liquid rubber plywood aquarium you don't have to paint it on, or deal with an epoxy smell as it cures. It is also a great liner for the aquarium that can flex if needed without cracking. The problem is finding a fish safe sealant to seal the joints if you want to use it in a plywood aquarium build. I'm going to show you the adhesion properties of three different caulks that may be good candidates. I'll give you my two scents on the safeness of the caulking, but you need to do your own research on this topic.

The three candidates I tried were Butyl Rubber, 100% Silicone Sealant for Plastic, and 100% Silicone,.

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I ran a bead down the side of two HDPE Panels and let the caulking cure. After curing, I put a weight on the panel to see how much weight it would hold. Not very scientific, but it gets the job done.

First up 100% silicone.

I was actually pretty surprise how much weight this caulking could hold. I've been told that silicone will not stick at all, yet it could hold the weight of a utility knife. I tried putting a tape measure on it and it failed. In this case, I believe it held better than what I thought it would do because of the rough texture.

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Here is the silicone that has taken the imprint of the roughness of the HDPE Panel which creates more surface area to stick too and prevents side movement.

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Now for the Butyl Rubber.

This caulking by far had the best adhesion properties of all three, but it takes a very long time for it to cure. I put the bead down the side and came back 24 hours later and the outside was hardened, but the interior was still soft. So the side would just bend when I put a weight on it. I let it cure for 3 more days, and it became more rigid. Here it is holding the weight of at 16' tape measurer. I also put a 25' tape measure on it, which it held for a while than started to bend. From my experience, this stuff takes a long time to cure which may be a fishsafe problem.

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Finally my favorite, 100% Silicone for Plastics.

Here is a picture of the caulking holding a 16' tape measure prepared the same way as the other caulks. It also held the 25' tape measure for a bit, but failed after I shook it a few times.

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Basically, I think any of these caulking would function well in a plywood aquarium where the water pressure is pushing the caulking against rigid sides.

My two cents on fish safe component of the caulking. The 100% silicone is try and true in the DIY community. Of course we all recommend using aquarium silicone for those of us that don't want to do our research or take chances. Butyl rubber is safer than PVC piping, however companies sneak other stuff in their product you have to be careful about. Like this white lightening has about 5% titanium dioxide which could cause problems for corals. Also, the solvent doesn't evaporate very fast, which could poison your fish as it leaches into the water. The 100% silicone for plastics has some pretty nasty stuff in it to help it cure to plastics. I think as long as it is fully cured your fish will be ok, and I've read some forums where they used this product without problems. I've decided to use this product in my HDPE Panel build, so I would suggest waiting tell I'm done with my build. I think my fish will be happy.

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This guy talked about doing a simillar thing with hdpe, not glass. Don't know if he built a tank in the end or not.

 

[CANAMONSTER]

Epoxy is tried and true. Almost all the time epoxy fails is from not building the tank properly or not applying epoxy properly. It's the best. Only thing better is polyurea

 

[robham777]

It sounds possible, but any bowing or flex in the plywood frame could result in cracked glass. It may not be much more expensive to just use thicker glass with euro bracing and forego the plywood altogether.

 

[J. H.]

It sounds possible, but any bowing or flex in the plywood frame could result in cracked glass. It may not be much more expensive to just use thicker glass with euro bracing and forego the plywood altogether.

If that is the issue, he could use many plates of glass and silicone them together, allowing for a lot more flex. Thin glass (1/8") is CHEEP, probably a lot cheaper than epoxy, or even FRP, especially if you buy small panes or use old windows.

 

[robham777]

If that is the issue, he could use many plates of glass and silicone them together, allowing for a lot more flex. Thin glass (1/8") is CHEEP, probably a lot cheaper than epoxy, or even FRP, especially if you buy small panes or use old windows.

That's true, I was looking at some pricing after I posted the comment. I was thinking maybe some thin foam board between the plywood and glass might help mask any imperfections in the wood.

 

[wannadivesteve]

Why not just seal the plywood inside and out? No humidity change, no expansion. maybe I'm looking at it too simply. People have been making epoxy coated plywood tanks for years. If you used glass, woe be it if the glass ever cracks for any reason.

 

[Jon_DK]

Hi all thanks for your comments and links.
Regarding the HDPE panels with silicone for plastic, this sounds interesting. But still there is this risk of toxic matters. Also the junction HDPE panel to front glass - i wonder if the plastic type silicone will adhere to glass.
I have tried to see if i could simulated stresses to the proposed construction... I am not a construction engineer so i quickly ran in to problems adding the correct material mechanical proporties for glass. It seems that glass tensile strength can vary a lot. Even small imperfection on the surface not visible to the eye, can weaken the glass. One thing however is clear. Glass has a very very high compressive strength. So as long as the supporting plywood is perfectly flat and rigid, the glass will not fail. Another thing that i didn't figure out is if the bond strength is affected by a static strecthing of the silicone.
I know that epoxy/glass fiber combo has proven as a safe way to do it. But applying the epoxy and glass-fiber, and avoid small bubbles etc.. is what I am uncertain about if I can do properly. And the fact that a wooden box will change its inner dimensions to some point (several millimeters). And using the epoxy that is stiff and non-elastic will transfer those stresses to the epoxy-shell. I like having a material with elastic properties to handle this in a somewhat controlled manner.
A sheet of foam/rubber to smooth out any imperfections on the plywood surface is a good idea i think.
I once had a friend who did glass aquariums for a living. He usually offered custom aquariums for a high and low price. The high price version would have a thick glass panel for the bottom that could hang freely without support (besides the support from the frame around). The low cost version was offered for customers who could provide a stand with support of the entire bottom surface. I am just thinking couldn't that trick be expanded to the side and back walls aswell ? He could sell large aquariums with thin glass in the bottom, without issues through many years. And I guess people building their own stands will not always be perfectly plane - so the glass must accepts some bending. Those diy stands usually consist of wood or blocks of concrete supporting a board of plywwod and some thin foam/rubber.

Well that was a long talk of not very much new.

 

[nzafi]

I have not built a plywood tank but have researched because I want to build one. Everyone is going to tell you that it will come down to how your support the tank. Multiple have had tons of success with pond armor. If you forgo the fiberglassing though, you need to find another way to strengthen the corners. Lots of folks add bracing across the back and sides with 2x4s for larger builds.

I was playing with the idea of the below epoxy because of its elastic nature but not sure if it really that much better than pond armor.
https://www.westsystem.com/specialty-epoxies/gflex-650-toughened-epoxy/

 

[wannadivesteve]

I'm still not understanding how a wooden box could change size IF you epoxy all exposed surfaces inside and out. Humidity could only cause expansion if it can reach the wood. If the box "expands" you'd still have change of support or pressure issues to deal with if you use glass or a similar liner. You'd want something completely flexible.