3M Collision Repair SOPs for Seam Sealers and Coatings

Customer Challenge: Our shop has always prided itself on creating an invisible repair. Yet the challenges associated with the many different shapes and types of seam sealer applications leave us feeling we could do a better job in this area. What can we do to improve?

Technical Tip: First off, it’s very important to choose a high-quality seam sealer and to properly prepare the joint – not just to achieve a lasting seal, but to ensure that it matches OEM appearance.  Seam sealers are developed for function and form, and both are crucial to the repair performance. Therefore, it’s important to choose the right sealer for the job. You can choose heavy sealers for vertical seams, controlled-flow sealers that limit waste, flexible sealers for joints which are subject to vibration, and many more. 3M recommends using pneumatic-type sealer applicators to produce consistent, continuous beads, and a range of applicator tips to most closely match the beads originally on the vehicle. Another key is to use masking tape to control the outline or footprint of the seam sealer, creating a defined edge.

Customer Challenge: We have seen conflicting information on applying a seam sealer over an etch primer. Is it okay?  

Technical Tip: The term “self-etching” has been somewhat loosely used and can mean different things in the collision repair industry. However, true self-etching primers contain an acidic component that can cause seam sealers not to adhere properly and to fail over time. Single-component or so called “self-etch” primers alone are also not a good choice for corrosion protection. Most OEMs use non-acidic primers as part of the many steps they take to prevent corrosion during coating and sealing procedures. Applying seam sealer to a high-quality two-component epoxy or urethane primer or OEM E-coat is the best option. If application to bare metal is the only acceptable method, choose a sealer designed, tested and proven for bare metal application. Dress and clean the welds. Sand and clean the bare metal – you should never apply sealers over dirty surfaces. Always follow your paint company’s procedures for primer application. If you use products not approved by your paint company, it will most likely lead to unwanted results.

Customer Challenge: Is it better to apply seam sealers directly to metal, or to painted or primed surfaces?

Technical Tip: In most cases, primer or E-coat is the preferred substrate for sealers. However, 3M has seam sealer technology using epoxy, urethane and modified silate polymer (MSP) chemistry tested and approved for bare metal and primer applications. Applying seam sealer to bare metal can present some unique challenges for painting. For example, you may find corrosion “hot spots” if primer and paint coatings are not applied adequately at the seam sealer edges (where sealer meets bare metal). Consider this closely during the repair plan, also taking into account that internal sealers are not subject to the same harsh environmental conditions as sealers used for exterior body applications.

An excellent example of bare metal seam sealer use is during a weld sealing operation (see SOP) that prevents water intrusion between panels. This includes spot welding while the sealer is uncured, which encapsulates the squeeze-type resistance spot welds in an air- and water-tight layer.