Bonding MDF

[wcsg]

Trying to see if my thinking is correct

If I need to glue (2) 2'x3' 3/4"thick MDF rectagles together to make a 1.5" thick sign to carve and paint I can just glue two sides right? This is for a under canopy sign with no direct exposure to sun or elements.

If so is there any prep I need to do prior to just adding some glue and clamping together beside making sure there's no dust?

[luke]

In the past, I used gorilla or yellow wood glue with no prep. It holds well but the down side is clamping time and then later carving into the glue layer. I recently switched to 3m Super 90 spray and am much more satisfied with the results. The bond is permanent and much tighter with less clamping than regular glues.

[wcsg]

Hmmmm spray glue huh? Maybe I'll try that one. Thanks!!

[benchmark]

Luke

I have a Bass speaker in my van after some time
I had a rattle so I opened it up, the access panel had a backing panel (all MDF) that had fallen off and punctured the bass driver...I later found out it was stuck with 3m Super 90 spray.

I would not fancy a lump of MDF landing on my head......Stick to good old exterior wood glue.


Paul

[wcsg]

Yeah, I have a 1000 watt 12" fosgate sub and that moves a lot of air. Made of 1" MDF and that's glued and screwed together, wow can't see creating a sub being held with only spray glue on the ends along with the pure weight of the thing, I'd be mad if I saw that lol. But maybe the spray would hold bonding to a large surface to surface area with just shearing fore to worry about and without having vibration to interfer.

But Luke have you done this with surface area bonding like MDF and it works okay. I've heard of using it to bond thin plex and alum to PVC and other subs for lettering and such

[joe]

EPOXY

[tuck]

Folks are gonna swear by whichever glue they prefer, but I haven't found anything that Gorilla Glue can't bond. It's a bit tricky to work with only in that it "expands" or "foams up" as it dries, so you have to clamp your pieces together real well. For pieces 2' x 3', I may clamp the edges tight and perhaps set a cinder block, old car battery or anything heavy in the middle. I call them "gravity clamps".

With Gorilla Glue, a little bit goes a long way. A common mistake is to use too much. Also, "air" is its' catylast. Be sure and put the cap back on the bottle when you're done or else it will set-up and harden in the bottle. The stuff ain't cheap, but Elmer's has a similar urathane-based glue out now called "Ultimate Glue" that's less expensive and just as good.

[brian_harnett]

I have had good results with Epoxy,Tightbond2 and polyurathane glue's
Epoxy workes the best for joints that are a poor fit it is the only glue of the 3 that retains full strength over joint gaps.

[chuck_reid]

I agree with Brian, epoxy is hard to beat. Just make sure you coat both surfaces to allow for absorbsion. Unfortunately it doesn't have any UV protection but a top coat of marine poly makes it nearly impervious to sun, rain, bugs etc.
For some interesting facts on epoxy and boatbulding, see Glen-l.com. The forum is similar to Shop-bot's and the amount of free information is incredible..

[luke]

I agree with all previous points made regarding 3m super 90 spray. It would foolish to build a speaker box using this spray. The 3m super series is designed as a high bond laminate spray glue. Its ultimate shear strength is 230psi. Erik’s sign surface area is 864 inches. My math is as bad is my grammar, but I calculate the total shear strength of Erik’s sign laminated would be pretty significant. Granted, other glues may have a higher shear strength but you trade this for clamping time, glue expense and tool wear. All listed details a side, it is really all about the right tool for the job. Are you laminating panels together or building a load bearing structure?

To digress a little from topic I thought this would be a good place (as long as were talking glue) to share something I learned while experimenting with Gorilla Glue. Everyone knows how messy this glue can be. For me, this made using this glue for repairing finished woods impractical until I realized this is just polyurethane. Polish the area you want to protect from glue overflow with PartAll Wax first. After gluing and during set up do not wipe the excess glue from this surface. When the glue repair is complete you will be able to pull the excess glue off in one bead with your finger.

[benchmark]

All

The speaker box itself was glued with normal wood glue, it was just the access panel that was two piece of MDF laminated together that was glued with 3m Super 90 spray.

[luke]

Yes Erik. I layup MDF to make plugs for fiberglass molds because it is so cheap and readily available. I found that when I prep sanded the plug It would develop valleys at the bond site because the glue was more resistant than the MDF surrounding it.