A Different Twist on the Magnet Gluing Process

motorcityguymotorcityguy Posts: 24
edited December 2018 in Vintage Speakers
Hope I’m not stepping out of line, for a newbie. But wanted to share a slightly different approach for the magnet gluing process.

Sometime back, I had my own bad experience with a shifted magnet on a Polk mid-woofer, so I decided to be proactive and perform the Polkie ritual of “preventative magnet gluing” on my multiple sets of Monitors. After spending a ton of time reading the varied experiences and opinions of others, it became clear that the two most favored products are JB-Weld and the water-based Loctite PowerGrab.

I tried the JB-Weld on one set of speakers, and found it to be very messy. It is difficult to apply and tough to clean-up. I know this is one of the reasons many have switched to the water-based PowerGrab product. But, with all due respect to several of the forum’s experienced contributors, I had serious reservations about using PowerGrab because it is specifically NOT recommended for two non-porous surfaces (like a magnet and steel).

One of the side benefits of my day job, is having contact with the engineering reps from industrial/automotive companies like Loctite. When it comes to selecting their best products for a job, I’ve found the engineers provide much better recommendations than the customer service folks at the 1-800 number. In this case, I explained the speaker problem to the rep, and explored what might be the best products for the job. Here is the 2-STEP process that we came up with, which I have successfully used on several sets of speakers.

STEP 1: Reinforcing the existing glue
Polk originally bonded the magnets to the metal parts using a very thin adhesive. As a result, the only gap between the assembled parts is due to minor flatness variation of the components and is nearly “zero”. In fact, the guys that disassemble and repair “magnet-shifted” speakers, invariably re-glue them with a thin anerobic product like Loctite 271 (red). This allows for a metal-to-metal fit.

In order to reinforce the existing original glue, it is necessary for an adhesive to be VERY thin, so it can wick into the existing gap between the magnet and frame. There is a product designed specifically for this. It is Loctite 290 (green), which is an ultra-thin product with exceptional capillary action (wicking). This product is so good at wicking, that it is designed to be applied to threaded fasteners AFTER they have been assembled, as it will work its way into the threaded joint. Like other Loctite threadlocker products, it dries in the absence of air. (More about this later.) As an added bonus, Loctite 290 bonds so well, it stated to not require pre-cleaning of surfaces.

So, the first step in my recommended repair is to apply the green Loctite 290 around the circumference of the magnet where you will later be applying the preventative glue/epoxy. Use it sparingly, let it wick in between the magnet and metal frame, then wipe off the excess. Ideally, let it dry overnight, just so any excess will not interfere with the next step.

One note about Loctite 290: It is not easy to find. Most auto parts stores do not have it and (BEWARE) those that do usually have expired material. I bought a tube at a local store, only to determine later it was 9 years old!! Loctite states that the shelf life of 290 is 30 months (2.5 yrs). I recommend buying it at a machinists or tool store, where the stock is likely to be much fresher.

STEP 2: Apply preventative glue
As mentioned, I was seeking something easier to use than 2-part epoxy, but it needed to be specifically suited for bonding metallic and non-porous surfaces. Here, with the help of the Loctite guy, I found an inexpensive and readily-available product. It is Loctite PL Premium urethane construction adhesive. It is the big brother of the water-based PowerGrab construction adhesive. The PL requires no mixing, and can be bought at big box stores in a small easy-to-handle tube. It does require solvents for clean-up, but unlike JB-Weld, it wipes off easily when wet. It is recommended to bond nearly anything, including metal, stainless steel, and other non-porous surfaces.

I wrapped the side of the magnet with masking tape, and used a finger to “tool” the PL adhesive into the area between the magnet and the frame. See photo. Once it’s been shaped, I removed the masking tape, for a clean and tidy job. The stuff is firm enough to stay in place (no dripping!) and skins over within a few hours. It won’t be fully dry and hard until the next day. A side benefit is that the PL will effectively block the air from getting to the Loctite 290 (STEP 1) and thus promote its anaerobic curing.

Ok, so that’s my two cents. I’m not suggesting this is the only way to glue magnets, or the best way. It’s just another alternative. Seems to me to work well, and doesn’t look sloppy. The products that are used should provide an effective 2-tiered solution: Reinforcement of the existing glue, and a preventive gluing to further guard against magnet movement. Sort of like belt and suspenders. ;-)


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