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I have attached two 3/4" panels of MDF together, so the end result is a 1 1/2"-thick panel. I need to reinforce it so that it does not bow, so I thought of attaching a 1/8"x1 1/2"x6' strip of steel along the edge. Simple research has introduced me to Liquid Nails and epoxy glue, but I would like to also hear from the pros at SE. I am not going to have any holes in the steel, so that eliminates screws. I am going to have to go the adhesive route with this one.

Edit: I left out one of the most important details, which is exactly what I am doing with this MDF. The double-layer MDF is not going to bear any load. Instead, it's going to be used as a custom door I am building. Since the MDF bows, I need to find a low-profile material that is even more rigid than the MDF. At 1/8" thick, this steel strip is very rigid. I just need to find a way to attach it so it holds as long as possible.

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I'm not a pro, but in this case I would go with a polyurethane construction adhesive. –  Jon Raynor Mar 25 '13 at 21:38
    
Have you calculated the weight of the door? A rough calculation puts it in the 150 pound range. –  mikes Mar 25 '13 at 22:48
    
I actually put it on the scale I use to weight myself and it comes in at around 90 lbs at 1 1/2" x 28" x 81". –  oscilatingcretin Mar 25 '13 at 23:00
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Because when I look at it from end to end, I can clearly see a distinct curve. I just tried attaching a 1/8"x1 1/2"x8' strip of aluminum to it with clamps and it helps a little, but the bend is so strong that it actually bends the aluminum along with it. I guess I am going to have to go the steel route after all. –  oscilatingcretin Mar 25 '13 at 23:08
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This would require a piece of MDF that will last for eternity. And eternity is a really long time. –  BMitch Mar 27 '13 at 2:16
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4 Answers 4

1) Use an 81" length of metal with a 'T' cross section. Cut a slot down the middle of the 81" edge of the panel to accommodate it. Dry fit, then assemble with construction adhesive. If desired, 'pin' the T-bar in place with nails or screws through the surface of the door.

... or ...

2) Use an 81" length of metal with a 'U' cross section and 1.5" outside dimension. Use a router to rabbit each of side of the 81" edge so that the U-channel fits onto the edge. If desired, 'pin' the U-channel in place with nails or screws through the surface of the door.

... or ...

3) Use an 81" length of metal with a 'U' cross section and 1.5" inside dimension. No routing necessary. If desired, 'pin' the U-channel in place with nails or screws through the surface of the door.

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Paint this door well because even indoor humidity will warp this thing fast. –  DMoore Apr 30 '13 at 17:14
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Unfortunately forever is simply impossible ;)

Without knowing what/how these planks are going to be used, it is difficult to provide a great answer. I have no real experience with liquid nail, but have used similar types of glue with success, but again, without the what/how you indeed to use these planks... I can't say too much for your case.

One issue with gluing to MDF is that you can create a great adhesive bond to the MDF but by MDF's nature, this bond is only made to the outermost 'layer'. Depending on how these panels are used, in particularly if they are under changing stress loads, it is posable for the 'layer' further deeper within the MDF to serrate from it self. That is, the steel would remain attached to the MDF, but only a thin layer of MDF that has serrated from the rest of the panel.

Depending on depth of the panels, you could have threaded rod welded to the inside surface of the steel and drill matching holes through the MDF panels and attach with a nut on the other side. This would provide a smooth finished end (the nuts on the back side could be counter sunk) but it really depends on the use/budget.

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How silly of me to leave out the most important detail. Edited my answer. –  oscilatingcretin Mar 25 '13 at 22:25
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My suggestion would be to embed square tube stock steel into the MDF and epoxy it in.

I recently did this exact thing with MDF subtop for my granite counters and uses a polyurethane glue to embed the steel.

Check out the pictures here:

Can I support a granite countertop overhang with embedded steel bars?

Square stock is rigid and cheap. Route a channel into the MDF and lay in the glue. Sandwiching the second sheet on MDF on top with glue in between should further secure it.

I assume this is a continuing of your sound-proof room product. So assuming your joists are secured above rubber absorbing standoffs, glue and screw the 3/4 MDF sheets into the joists.

After that, route out channels for the steel square stock. Thoroughly vacuum and wet-rag dust it for good adhesion. I highly recommend you have a helper hold a shop-vac right with you as you route the channels in the MDF. MDF dust gets everywhere.

With the channels routed, lay in a large bead of glue, then lay in the steel. Then another bead of glue on top of the steel and use a throw-away putty knife the spread and flatten it into the side-pockets of the channels, etc.

After this, stagger, glue and screw the second MDF sheet on top. I would use a decking screw for this and turn the torque down on your drill do just what you need.

This will take you some time, I like to use the torx decking screws because they're easiest to drive reliably.

All said and done you'll have a 1.5" rigid, steel-reinforced panel.

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Your answer is pretty much dead-on with what I plan to do after this door. I am basically looking for a way to reinforce this door as it exists now. Since I am convinced it's impossible, my next goal is to use three layers of 1/2" MDF, route out channels in the middle layer, and fit them with 1/2" square or round stock (I hear round is even more rigid). And, yes, MDF dust is a mess. I routed a 1 1/4" channel into the edge of this door so I could fit a length of steel into it and the stuff was all over the place =\ (and it didn't work) –  oscilatingcretin Apr 30 '13 at 17:02
    
Well, hmmm. I just got back from Menard's checking out their 1/2" square steel tubes and they're not as stiff as I thought. I can actually flex them with ease. Where did you get yours? –  oscilatingcretin Apr 30 '13 at 22:04
    
I got mine at Lowes. Consider that you're to compare the rigidity of the steel square stock to the equivalent shape in MDF. You can also get aluminum square stock from many providers online. –  Matthew Apr 30 '13 at 22:07
    
I've decided I am doing this, but with a twist. The larger the square stock, the more rigid. With a 3/4" panel, I can only practically fit a 5/8" square (this leaves 1/8" between the stock and the door's outer surface). I want to use 1" stock, so I am going to get 3/4" MDF, put 1/2" MDF on top, route two 1" channels for my 1" stock, then cover with 1/4" hardboard. Now that I have three layers, I will be able to have two layers of GG. I will order my materials today or tomorrow and may start this weekend. I will post pics to this question when I am done. –  oscilatingcretin May 2 '13 at 11:16
    
@oscilatingcretin lining up your channels may be more of a challenge than you bargained for. At this stage you might as well have square stock welded into a reinforced and rigid shape. –  Matthew May 2 '13 at 14:19
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Liquid Nails comes in various formulations, some better than others. Of this type adhesive, polyurethane based of any brand will give the best bond, as Jon Raynor suggested. I don't have hard data, and again it varies by formulation, but epoxy should give an equal or better bond than polyurethane, and is less susceptible to creep deformation. Epoxies that require heat curing will be stronger, but are difficult for DIY sorts to use correctly. Longer cure epoxies are probably slightly stronger than the common 5 minute type. Unless you can find bulk quantities, the cost will be rather high.

As pdd suggests, a strong bond is not much use when the internal MDF bonds fail, and you end up with a strip of steel and glue with a thin layer of MDF attached to it. I strongly suggest you consider some mechanical connection in addition to adhesive. Screws in to the edge of MDF panels are marginal at best, but still better than nothing. Or some sort of welded stud scheme may work if screw heads are simply wrong. Or perhaps use Tee sections with the stem inserted into a cut groove in the panel edges. If you then through bolted or screwed that stem, you would have an incredible mechanical connection, along with added stiffness (and weight). The fasteners could be used for decorative effect, or countersunk and plugged to hide them.

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