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May 26, 2023

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We went mano a mano with the popular adhesive.

Just how tough is J-B Weld? I wondered about that recently when I found myself at a Tractor Supply Company buying plant and garden materials and ran across a rack of the well-known repair adhesive. That was about the umpteenth time that thought crossed my mind when I found myself shopping for something else. You run across J-B Weld products everywhere you go, it seems. Investigating that fact, I learned on the company’s website that its products are sold at 50,000 locations around the world. I bet half of them are in the state of New Jersey.

I may be kidding about that, but I’m not kidding about the curiosity that this material has occasioned. Go online. People repair everything under the sun with this stuff. Most of them are small fixes to things on cars, signage, gizmos. Some of these repairs look kind of janky and might have been better executed with welding, bolts, or screws. But who am I to say? The owners seem to be happy with their results.

My encounter with JB-Weld at Tractor Supply was an itch I had to scratch. I bought two types of J-B Weld and brought them into the Popular Mechanics shop in an attempt to find out, just how tough this repair material really is.

Before I get to that, you might find some background helpful.

In its cured state, epoxy is a thermoset plastic (it cannot be melted by heat and reformed). It starts out as two chemicals (or complex chemical substances) that are mixed together, driving a reaction that turns them from soft putties or syrupy liquids into a hard plastic. Although epoxy has been under experimental development for more than 100 years, it wasn’t successfully developed for commercial purposes until the late 1940s. By the mid 1950s there were several companies producing it, and J-B Weld was founded in 1969. The company’s first material was a simple epoxy putty used in truck repair. It described this flagship as “tougher than steel.”

We’ve got nothing against epoxy, but that claim, “tougher than steel,” is an exaggeration. You can’t take epoxy and form it into a drill bit or a cutter for a milling machine. Still, epoxy is tough stuff. For repair purposes, it has a lot to offer. It’s versatile, and it can be successfully applied by amateurs.

So, scratching that itch, I bought one package of Plastic Weld and another of the company’s flagship Cold-Weld Formula Steel Reinforced Epoxy. I used the adhesives to glue together steel, wood, PVC plastic and combinations of those materials. We let the adhesives cure, then we tried to break the samples apart.

Disclaimer: this isn’t lab-based analysis. It’s rough and ready, seat-of-the-pants testing. We read the product directions, prepared our samples by roughening the bonding surfaces with 60-grit sandpaper. We rinsed the surfaces with clean water, let them dry, then mixed the adhesives per the package directions. We applied the epoxies to our test materials and set the test samples aside for 24 hours to allow the adhesive to cure.

Finally, we broke the samples apart. Our results are documented below. Our primary test instruments (to call them that) were a gigantic machinist vise, and a whopping 32-oz. solid steel Estwing ball peen hammer. We also used Channellock pliers and locking pliers.

In general, we have to commend J-B Weld for making an adhesive that mixes easily, applies smoothly, and forms an incredibly tenacious bond. Some of our tests show that the bond strength of J-B Weld easily exceeds the strength of the material to which it’s bonded.

Test sample: 1/2-inch PVC pipe bonded end to end with Plastic WeldTest method: hand pressure, plier twistingComments: The sample was impossible to break with hand pressure but snapped off easily with a pair of locking pliers fastened to the end of the pipe. Such a bond would be more than strong enough to support a small sign, for example, used for indoor purposes. The lack of bonding surface could not withstand the torque applied by locking pliers.

Test sample: Kiln-dried poplar blocks joined face grain to face grain with J-B Weld Original Cold-Weld Formula by applying the adhesive and pressing the blocks together with hand pressure.Test method: Clamp the test blocks in a machinist vise and deliver repeated blows to end grain and face grain with a ball peen hammer.Comments: We were unable to break the bond created by J-B Weld. Repeated side blows to the end grain sheared the block. When we repeated the test delivering blows to the face grain, the block sheared again but the bond remained intact.

Test sample: Low-carbon steel joined to form a T. Surface and end roughened by grinding with 60-grit abrasive. J-B Weld Original Cold Weld applied to end and face of T and pressure applied by hand. We also used an additional build up of epoxy to form a fillet around bond joint.Test method: Clamp the T in a machinist vise and pry apart with pliers. Strike T on anvil face.Comment: The length of the vertical protion of the T joint was not long enough to permit us to apply sufficient torque with pliers to break the joint. Only a sharp strike on the face of the anvil was such to cause joint failure.

Test sample: Original Cold-Weld Formula applied to both bond surfaces of low-carbon steel sample pieces and squeezed together with hand pressure. Test method: Clamp lapped piece horizontally in machinist vise and deliver end blows with ball peen hammer. If needed, clamp sample in vise vertically and deliver blows to face of sample.Comment: This was by far the toughest sample to break, indicating the value of bonded area relative to the stress being applied. Several hard blows (nearly as hard as I could possibly swing the hammer) mushroomed the end of the sample but failed to break the joint. The lap failed when the sample piece was turned vertically and a very sharp blow was delivered horizontally to the mushroomed end, causing the upper piece of the lapped joint to break free.

Test sample: Hole was drilled into a poplar block and 1/2-inch PVC pipe was bonded to the block with Plastic Weld. Test method: Strike the poplar mallet head on the vise’s anvil, twist handle by hand, pry handle with locking pliers, hammer the handle.Comment: Several severe blows of the mallet on the vise anvil could not break the handle-head joint. Clamping the head in the vise and twisting the handle by hand (and even locking pliers) had no effect on the handle-head joint. The joint only failed from several powerful blows to the handle, close to where it joined the head. Our takeaway here is that joint strength is increased whenever you can combine the mechanical advantage of an interlocking joint with adhesive.

You’ll get your best results with J-B Weld by applying it to a clean, roughened surface. Roughness (within reason) counts for a lot. First, it increases the bond surface area. When seen under magnification, a roughened surface has more area compared to a smooth one. You can picture this pretty easily if you consider, say, the surface of the moon versus the surface of a marble the size of the moon. Although both have theoretically the same surface area, the irregularities on the moon’s surface would provide increased surface to which to apply adhesive.

Second, when seen under magnification, the roughened surface provides what engineers refer to as "tooth." That is, the profile of a surface roughened with abrasives provides saw-blade-like teeth that mechanically lock with the adhesive around them. We’ve oversimplified this to make the point about the need for roughening a bonded surface. To understand this phenomenon more fully, particularly as it relates to epoxies, see this article, "Need to Improve Bond Strength?"

Also, adding reinforcement can increase the strength of the joint. Bonding additional pieces of wood or metal over an epoxied joint can mechanically stiffen the joint and help it to better withstand the variety of forces that act on it.

Finally, whenever possible, you can improve your odds of bonding success by making use of a mechanical joint that works with adhesive. Such a joint increases the surface bonding area and reinforces the joint by distributing the adhesive in three dimensions. The pipe socketed into the mallet head is an example of this that’s easy to do (and to understand). In the case of the mallet, we were sure that the head would come flying off on the first hit on the vise’s anvil. Nope. Repeated hits failed to break the joint. In fact, we were concerned that the mallet head would fail. Combining mechanical bonding (a socket joint connecting head and handle) plus the chemical joining power of the adhesive provides a strong one-two punch of fastening power.

Roy Berendsohn has worked for more than 25 years at Popular Mechanics, where he has written on carpentry, masonry, painting, plumbing, electrical, woodworking, blacksmithing, welding, lawn care, chainsaw use, and outdoor power equipment. When he’s not working on his own house, he volunteers with Sovereign Grace Church doing home repair for families in rural, suburban and urban locations throughout central and southern New Jersey.

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