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Proper Seam Sealing Procedures Key to Eliminating Paint Issues

( 07/17/2018 ) Written by: Douglas Craig
One common complaint that is shared with me on a frequent basis is that tape pulls paint off the sealer when a 1k seam sealer is used. While unfortunate, this is a process, not a product issue. The sealer has not fully cured and the bond strength between the primer and the sealer is weak, thought it does seem to improve with time. Applying masking tape over this type of situation will lead to failures. By better understanding proper application, this problem can be avoided. 
To begin, seams that will be subject to some type of taping (typically closure panel hem flanges), or in-use abrasion (door hem flanges, exposed pickup bed joints, weatherstrip contact, or similar) should always use a 2K sealer. The 2K materials, due to their very short cure time compared with ambient curing 1K materials, are significantly more robust and can tolerate “handling” plus the curing reaction is complete when the paint system is applied. Other tips for success: 
  • Testing: all paint systems should be tested by the end-user over sealer systems to ensure compatibility. Simply applying sealer and paint to a scrap part in the fashion as the products will be used in “production” will be adequate. Final system performance can be reviewed, post full cure, with masking tape, finger nail abrasion, etc., to suit the end-user.
  • “Paintable” indicates that paint can adhere but does not guarantee how well the system will perform in use. Since 1K sealers are all flexible, even a perfectly adhered paint system may “fail” when exposed to abrasion or impact. Flex agents can be added to the various paint layers to improve this situation. However, this typically does not occur except with fascias, and even then, the consequences of improper procedures can be seen on a daily basis in traffic.
  • “Direct to metal” indicates that the material may be applied to uncoated, properly prepared, metals. “Properly prepared” means clean, no oils/grease/mud/mold/etc., and the surface has some tooth to grab hold of. Polished aluminum is going to provide very little for the product to grab hold of while sanded or scuffed surfaces will.
  • “Not curing” is another common complaint with 1K products and it must be qualified as to whether the sealer is not curing and is still soft, or if the paint is soft or tacky. Unless there is a product issue, which is unlikely, a non-curing sealer can be traced back to time/temp/humidity, which would also include being encapsulated and having no exposure. Soft or tacky paint over the sealer would indicate an incompatibility which should have been identified during screening. 
  • Another common incompatibility situation in which the adhesive/sealer is often wrongly associated with failure is the use of polyester-based primers and fillers. Catalyzed polyester primers may have reactive failures over and under most urethane and epoxy products, while catalyzed epoxy primers will robustly bond with epoxy and urethane products and also with steel, aluminum and composite substrates. LORD recommends that polyester-based products are not used in combination with our seam sealers or plastic repair materials.
To summarize, most of the “issues” we deal with on an ongoing basis can be avoided by ensuring the right product is selected for the application, as well as adhering to the defined process. LORD Corporation’s Fusor family of products are paintable and there are few compatibility concerns. However, no matter the product selected, adhering to the right process will eliminate the majority, if not all, concerns.
Douglas Craig

In his structural adhesive technical service role at LORD, Doug is responsible for providing guidance and repair solutions to repairers, OEMS and insurers which incorporate the proper materials and methods. Doug's previous role, as the Collision Repair Manager at Fiat Chrysler Automobile, was focused on all topics related to collision damageability and repairability.

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