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Understanding the Procedures for One-Component vs. Two-Component Seam Sealers

( 02/05/2021 ) Written by: Douglas Craig

Seam sealers are an essential part of duplicating factory appearance while making a repair to automobiles and industrial heavy-duty trucks, buses and trailers. Currently, repair shops can use either one-component (1K) or two-component (2K) seam sealer, further split by products which require a 2K primer and those known as direct-to-metal or DTM. What are the differences?

A one-component DTM system uses a caulking gun to apply the product onto the car where it begins to cure. Most one-component sealers cure via a reaction with water in the air. A primer is not used beforehand when the material is DTM but there are products in the marketplace that do require a 2K primer. Many technicians and shops always apply primer even when using DTM products as this is considered the ultimate repair procedure.

A two-component seam sealer involves the mixing of two different materials to create the sealer. The two-component system utilizes a cartridge system that contains two parts - A and B. As it’s being extruded, it will go through the mixer to combine the parts. When it reaches the car, it is mixed and is now having a chemical reaction that will activate curing. As with 1K sealers, 2K seam sealers may need a 2K primer first or may be DTM. Certain applications require 2K priming regardless of the seam sealers claims. Areas like roof ditches which are exposed to flexing from torsional body twist must always be primed.

1K and 2K products have significant cure rate differences. Because 1K seam sealers generally cure via a reaction with water in the air, their cure time can be affected by the weather – they cure faster in humid weather and can really slow down in the drier winter months. 2K products go through a chemical cure due to mixing A and B together and do not rely on the weather for curing but can be impacted by the temperature – they speed up in hot weather and slow down in cool.

The 1K products are generally “skinned over” in 25-30 minutes and fully cure in roughly 24 hours, depending how thick it is. How fast a two-component sealer cures will depend upon the formula. Generally, their work times are 3 to 20 minutes. After a half-hour most every 2K seam sealer is firm enough that you can’t make a mark on it. While it may not fully cure for 24 to 48 hours, you can touch it without damaging it and with most the paint can be applied to it. 1K sealers on the other hand are very fragile for several hours and must be handled carefully or they will be damaged.

The proper process and procedures are necessary for all types of seam sealers for successful usage: 


  • Sand any paint edges before the seam sealer goes down so you’re not working around fresh sealer when prepping for paint.
  • Clean! Make sure there is no dust, dirt or rust on the shiny metal. 
  • Apply masking tape, if desired, to define the size of finished bead. Important here is to remove any tape before the sealer begins to set up.

Steps for One-Component Seam Sealer:

  • Place into caulking gun and prepare per manufacturer’s directions.
  • Apply sealer.
  • Sealer can be spread with a spreader or other tool.
  • Sealer can be brushed for a brushed appearance.

Steps for Two-Component Seam Sealer

  • Prepare the seam sealer cartridge ahead of use.
  • The first step in preparation is leveling by placing the cartridge in the applicator gun and extruding until you see the material, both the A-side and B-side, after which the material is wiped off the cartridge and the mixer is installed. This is an important step referred to as leveling. There could be a couple millimeters more of material on one side, so leveling ensures both A and B side come out on ratio. 
  • The next step is, when ready to apply the seam sealer, extrude a mixer’s length sealer as waste ensuring that A and B are mixing properly – this is the purging process. If striations or streaks are visible, the mix is not even. Extrude more material until it is evenly mixed.
  • Leveling and purging errors are where almost all product failures begin. 
  • Apply sealer.
  • Seam sealer can be spread with a spreader or other tool.
  • Seam sealer can be brushed for a brushed appearance.

Types of Seam Sealers

  • Non-Sag: Thick and holds texture such as brush marks and is often referred to as “full bodied”.

  • Controlled Flow: Thinner in viscosity. They are still thick but will smooth out when tape is removed.

  • Self-Leveling: Very thin and will flow out to an edge. One use is the roof ditch where the side of car meets the roof.

Additionally, every product has technical data sheets and a training program to show the proper use. Use these resources to train all technicians in the best procedures. It may seem simple, but don’t forget seam sealer manufacturers always have a customer service phone number, website and email for questions. The manufacturer is the most underused resource for a technician.

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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