An adhesives checklist

June 28, 2021
There are a few common problems that always top the list when working with adhesives. Here, we take a look at the top eight, examine why they occur and provide solutions.

Adhesives of various types are used daily in the collision repair arena. While the majority of products will perform as stated, sometimes users experience underperformance or even adhesive failure. These failures are often attributed to “bad glue” and result in phone calls to the adhesive manufacturer’s technical hotline. Rarely, however, is actual glue chemistry at fault. Most problems with adhesives can be prevented by following manufacturers’ recommendations. 

There are a few common problems that always top the list. Here, we take a look at the top eight, examine why they occur and provide solutions. 

Problem #1: Out-of-date material 

Out-of-date material is responsible for most adhesive failures. The solution is to make sure, before applying, that the material is within its usable life. Sometimes this is a simple, single step, since many products have their life stated in months from manufacture along with a manufacture date. Some products do not include a clearly written date, using an obscure code instead. The temptation can be to skip the step of verifying the material’s use-by date. But it is always worthwhile to call the manufacturer’s technical hotline and have them check the code. 

Problem #2: Poor product mix 

Two-component, or 2K, materials require a precise ratio of parts A and B to achieve their designed qualities. These ratios are achieved by using the level-and-purge process of getting proper flow through cartridges. While it is easy to forget or cut corners on the level-and-purge stage of prep, it is critical and should be performed as follows: 

  • Cartridges are generally “back filled” and then the pistons are installed. The components may be slightly overfilled, so to bring the cartridge to a usable point the technician must put it in the applicator, with the plugs or cap removed, and pump slightly until both A and B portions exit the cartridge. When both sides are flowing, clean off the pump-out and mix the product together. 

  • When ready to apply the material, install a mixer and then extrude a bead equal in length to the length of the mixer. Look at the bead to determine if the mix is homogeneous (well mixed). If it is not, extrude more until a homogenous mix is obtained. Test beads can be mixed with the product from leveling so they all cure and can be disposed of safely. 

Problem #3: Shop temperature 

Ambient shop temperature, product temperature, and vehicle or component temperature directly impact work times. For every adhesive, the work time is stated at a certain temperature. (The same is true for the adhesive’s clamp time, recommendations for sanding, etc.). If working with components that are outside the given temperature range, a generalization is that if the component is 20 degrees warmer than stated in manufacturer recommendations, the work time will be cut in half, and if the component is 20 degrees cooler, work time will be about 50 percent longer. For example, if a product has 5 minutes of work time at 70 degrees F, at 90 degrees work time would be reduced to about 2.5 minutes. Conversely, if the temperature is 50 degrees F, the work time is extended to about 7.5 minutes. Many technicians will have noticed these variances in work time but may not have been sure what caused it. 

Problem #4: Improper storage 

Storage temperature and stock rotation (which ties in to item number one on our list) are the potential pitfalls. For most adhesives, the ideal storage temperature is 60-80 degrees F. Note that storage location can affect the storage temperature: exterior walls may conduct heat into (or out of) the facility, making the actual temperature the adhesive is exposed to much hotter or colder than it would be at the shop’s interior. Every shop is different, so identify the best storage spot and consistently keep product in that location. When it comes to stock rotation, “first in/first out” is the rule of thumb. 

Problem #5: Substrate preparation 

Grit, melted plastics and surface modifiers can all interfere with adhesive performance. Therefore, the degree of surface preparation undertaken by the technician is important. OEM procedures for preparation must be followed. As a general rule: 

  • For sanding, when OEM recommendations are unavailable, it is advisable to use the least aggressive grit possible so as not to thin the substrate.  

  • Plastic repairs require a pre-cleaning step so that surface impurities do not get worked into the plastic during grinding and sanding.  

  • Grinding operations create heat, and for plastics with integral mold release agents the heat can infiltrate the plastic and cause the mold release agents to seep out. Therefore, it is important to always perform a low-speed sanding to remove any “melted” or greasy” plastic. Follow up with a very light application of the proper surface modifier, with adequate flash time, before applying adhesives. 

Problem #6: Improper finishing 

Urethane and epoxy repair products are not compatible with polyester-type fillers, glazes, and primers. Read the product technical data sheet (TDS) and standard operating procedure (SOP) to understand the proper steps. When in doubt, test the repair product on a scrap panel. Also, do not forget UV cured primers—the repair must be treated like raw plastic and the proper paint system adhesion promotor should be used or the paint will fail. Always refer to paint manufacturer’s system instructions when painting plastics and treat repair adhesives as if they were plastic. 

Problem #7: Product selection 

Proper product selection (along with following SOPs to the letter) will eliminate most issues before they even begin. With so many repair products and substrates in the market, sales representatives can be a great resource in determining the best adhesive for a given project. Or, once project is under way, just call the technical service hotlinethey are waiting to help! 

Problem #8: Test before using 

Thoroughly test “new” applications or products. Using scrap panelsapply the product, complete the finishing process, and apply the paint system. Then review product performanceThis does not have to be a rush item—testing can be performed in conjunction with other jobs.  Testing a product before using it directly on a customer’s car eliminates future rework by proving the process beforehand – a few minutes now can save many minutes later. 

It’s been said before, but it bears repeating: following directions is the key to success. Even though shops are time-constrained and reading technical literature—or performing prep and test steps—may seem like something that can be sidestepped, all too often skipping such steps leads to poor outcomes. In the vast number of cases, when performance issues are encountered in the shop, it is because one of the top eight issues hasn’t been adequately addressed. By following best practices, the rate of success when bonding with adhesives will be nearly one hundred percent. 

About the Author

Douglas Craig

Douglas Craig is the Structural Adhesives Applications Engineering Manager & Collision Repair Industry Liaison for LORD Corporation.

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