Questions for all you plywood guys

[Fiziksgeek]

Hello everyone, just joined, but have lurked for a while. About 3 years ago, I tore down my 210 in-wall reef tank and moved. I sold everything, as I figured it would be a few years before I got back in the game again. Well, I have moved back to my native area, have been at a great new job for several months now, and bought a new house back in July. I am getting the itch!!

I am thinking of a basic rectangular tank in the 500+/- gallon range, or perhaps an L-shaped tank going around an outside corner.

I have a couple of ideas I am looking for experience opinions on.

1. Instead of using 2x lumber for framing, I am thinking of using plywood strips, 2-3" wide. They are relatively easy to cut, should be almost perfectly straight (compared to the typical 2x lumber), and would be more dimensionally stable than solid wood also. Though not as strong, I could simply use tighter spacing.

2. I am considering trying to build the tank without any fasteners (no nails, screws, etc. My thought is to use the West system epoxy as a glue. Put down the plywood interior sheet, use epoxy, perhaps thickened a bit as glue on the plywood framing, put down the exterior plywood sheet, clamp good, pile on sand bags, buckets, whatever I can find that it heavy, and let dry.

3. I want to build all the panels, fiberglass and epoxy them, then assemble into a tank. As a final step, fiberglass/epoxy the joints inside and outside. It would seem that this would make the fiberglass/epoxy work much easier. Everything could lay flat to work on right up until the end. Limits the amount of effort in moving around a tank this large myself.

Sorry for the long winded post. Hopefully I'll get some good ideas out of you guys. Thanks!

 

[the_deeb]

Welcome to the forum. In response to your questions:

1. Plywood should be fine. It resists warping better than typical lumber so you may in fact be better off doing it this way.

2. I would advise against this, unless you were a very skilled woodworker/boatbuilder with a lot of experience working with epoxy. Epoxy alone might be ok for face joining pieces where you have a large surface area. The problem with butt-joining plywood ends is that the open ends suck up a lot of epoxy and weaken the joint. Even if you thoroughly wet out the joint before applying your thickened epoxy, I'd be worried about a simple butt joint holding up to the pressure of a tall tank without additional fasteners.

If you have the skill and equipment to incorporate some fancy joinery, like a rabbet, box corner joint or dovetailed edge, this would offer a little more strength than a simple butt joint and you might be able to get away with that without additional fasteners (though plywood doesn't lend itself very well to this).

If you really want to try this without fasteners, the best solution would probably just be to use loads of fiberglass. A simple butt joint with a thick epoxy fillet and several layers of wide layers of fiberglass overlapping the joint would be very strong.

3. This sounds like it would be a fine approach.

 

[pengu13]

I got a similar idea from west system when i emailed them about building a tank. really think it would work out with enough fiberglass and epoxy and i also agree doing all the sheets before assembly would make it easier but i don't think you could get away with not using some sort of fastener. i just don't think filleting the corners is enough alone i'm guessing you checked out their web site if you went this road i would use their instructions like this http://www.westsystem.com/ss/bonding-hardware/

 

[CJH]

What is the goal/purpose for a no-fastener tank? Is it for appearance reasons? I may have some suggestions depending on your answer. In the meantime I won't make assumptions and just address your specific questions.

1) Strips of plywood will be fine. This is what woodworkers use when they build large torsion box assembly tables that need to be perfectly flat. Remember that the strength of said torsion box is more related to the height of each strip than it is to the thickness. So in other words, use the 3" wide strips instead of the 2" wide strips if you can handle the extra thickness.

2) Epoxy is often used when builing large torsion boxes due to the ability to specify a product with a longer open working time (i.e. how long you have to work with it until it cures). It is tough getting all those strips of ply perfectly in place so you need an adhesive with a long working time. A previous poster mentioned the cut edges of plywood soaking up epoxy. While this is a concern do understand that if you get the right amount and viscosity of epoxy in that joint the surface area of the cut ends will only increase the mechanical bond of the epoxy. So this is both a hinderance and a benefit.

One problem with this type of assembly is that all of those plywood strips are going to want to wander on you when setting up the basic structure. The epoxy will act a lubricant and it will be very difficult to line everything up. Again back to the torsion box example, woodworkers often use small brads to hold everything in place until the epoxy or glue sets up.

But since you're considering a fastener-less construction, I highly recommend that you cut shallow dados in the interior/exterior face panels. this will keep the strips from wandering and also give the epoxy an area to pool up which will only help control the amount of epoxy in the joint. Dados will also make it possible to use thinner strips of ply. The thinner they are the more likely they are to wander.

Depending on how big this tank will be, I'd at least look into making a large vacuum bag for a clamping system. Much better then sand bags, etc.

3) I like the idea of epoxying each panel before final assembly while they're laid out flat. Much easier to control the thickness of the epoxy across the entire panel when it's flat instead of vertical. And easier to work on the bottom of the tank without the sides being present.

Just make sure that you do that final assembly within the recoat time of the epoxy. If you wait longer than that you'll have to scuff sand the epoxy to get a mechanical bond. If you assemble within the recoat time you'll get an actual cross-linked chemical bond. Talk to West Systems or whatever epoxy company you go with about this.

Also talk to them about the best strategy for the fiberglass reinforcement. Last time I talked to West Systems they recommended no fiberglass inside fo the tank. But in your case it will probably be a good idea, at least on the interior corners.

Again, I'm curious to hear the specific goal for not using fasteners and if you're open to suggestions to work around that. I can understand not wanting any inside of the tank but this would be avoidable by using pocket hole screws, for example. But there I am making assumptions...

 

[Fiziksgeek]

Hey guys, sorry for the slow response, its been a busy holiday weekend so far!

I guess I am thinking about not using any mechanical fasteners simply because I have seen a couple of build threads where people have experiences bubbling or defects forming around the fasteners (usually screws). I don't know exactly why this would happen, but if a defect formed, and water gets in, well, you know the rest...If I could avoid using screws, I could guarantee no problem because of them! Perhaps there was some other defects in the tanks, allowing water to get to the screws....who knows.

Matching dados in the panels sounds like a definite a must.

I thought epoxy might be easier than using wood glue. If I were to use fasteners, what would you guys recommend...a nice coated deck screw would be my thought?

I have a large roll of 6oz fiberglass cloth (50" wide, maybe 20 yards), which I thought would be perfect for this tank. Maybe I'll use it for the field, and pick up something stronger for the corners. Because of time constraints, and space, I would probably make up one panel at a time, and set it aside.

I appreciate your thoughts, feel free to keep commenting!

 

[CJH]

If I could avoid using screws, I could guarantee no problem because of them! If I were to use fasteners, what would you guys recommend...a nice coated deck screw would be my thought?

So it sounds like you only want to avoid using screws inside of the tank that would have to be sealed over? That sounds reasonable. But to be fair, there are an awful lot of tanks that have held water for decades that have screws on the interior.

But back to your idea, I would still use screws along the edges where you're just drilling through the face of one panel and into the edge of another. I have these really nice square drive stainless screws I bought years ago. Very nice and I've never found anything quite as good. But it's what I'd be looking for if building a plywood tank.

You could then use pocket hole screws to fasten your plywood support strips to the sides of the tank itself. Look up Kreg pocket hole jigs/screws to see what I mean.

Then when you attach your outer skin you can just drill through it normally and into the plywood strips. This would mean no screws inside of the tank.

Technically you could use pocket hole assembly for everything but you'd need some sort of small 90* drill to get in the cavities formed by the two skins over the plywood strips. More trouble than it's worth unless you're trying to hide all of your fasteners.

Pocket holes and screws are said to make for a stronger edge joint than typical screws. I have a Kreg pocket hole jig and while I am impressed with the strength, part of me is skeptical that they are significantly stronger. Very nice way to hide screws and screw holes, though.

 

[Fiziksgeek]

@CJH--I know you can get 18-8 or 316 stainless screws, square drive, up to about 6" in length from a place like McMaster, but they get very expensive quick! Its about $10 for 50-#8x2" screws.

I am going to be thinking/planning for a while here...Thanks Again