What's your epoxy primer knowledge?

April 20, 2016
Epoxy and DTMs (Direct-To-Metal Primers) can serve an important role in the repair shop by helping to prevent corrosion and promote bonding of materials. 

James Meyer, Senior Collision Training Administrator, was the lead author of this article.

Epoxy and DTMs (Direct-To-Metal Primers) can serve an important role in the repair shop by helping to prevent corrosion and promote bonding of materials. They are the closest match in the aftermarket to the Electrophoretic-Deposition Primer — or E-coat — that is applied during Toyota’s manufacturing process. So let’s tackle when they should be used and uncomplicated ways to apply them.

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When to use Epoxy and DTM Primers to ensure a high-quality repair

The easy answer to the question of when to use Epoxy and DTM Primers is every time you plan on applying paint coatings or body filler to bare metal. However, it isn’t always that clear cut. It is no secret that exposed metal is susceptible to rust over time — Epoxy and DTM primers help prevent this by sealing the metal and protecting it from corrosive elements. In body shops, we perform a wide variety of body and frame repairs, not just metalworking repairs.

When we repair frames and unibody structures on pulling racks, clamping witness marks and repair areas can expose bare metal. These need to be primed and coated to match the surrounding area. Epoxy and DTM primers will ensure a solid base for corrosion protection and that your topcoats will adhere as well.

During welding operations, the heat from GMAW/MIG (Gas Metal Arc Welding/Metal Inert Gas) welding will burn and vaporize the zinc-rich weld-through primer at the weld location. These areas should be properly cleaned and coated inside and out with Epoxy or DTM primer to provide corrosion resistance to heat-affected metal.

On exposed metal that will not be welded, Epoxy and DTM primers provide a higher level of corrosion protection and adhesion than weld-through primers. Don’t be tempted to use weld-through primers as a coating on bare metal to ward off corrosion. Zinc-rich primer coatings will protect against corrosion between STRSW (Squeeze-Type Resistant Spot Welding) welded flanges, but they can also compromise adhesion of subsequent adhesive, seam sealer and paint coatings if applied unnecessarily.

Replacing bonded glass may also require priming. For example, when bonding a back glass because a quarter panel has been replaced, bare metal will be exposed at the welded bond flange. Be careful to prepare the bond flange surface properly and under no condition should you apply body filler or refinish the bonding surfaces. Use an Epoxy or DTM primer on these surfaces to ensure proper adhesion of the glass bonding material.

And for common body filler repairs, it is recommended to apply epoxy primer to the metal first, and then apply the body filler. Toyota body panels have zinc coatings that get damaged and removed during metal work. And as mentioned before, zinc-rich coatings will protect against corrosion, but they tend to resist paint adhesion. This is crucial on exterior body repairs. However, it is also important to be familiar with your body filler’s chemicals. Most filler hardeners have a high peroxide (oxygen generator) content that can react with environmental factors and cause rust to develop in minute voids under the filler. Without an epoxy primer foundation, it is possible for the metal to be vulnerable to corrosion.

Proper application of Epoxy and DTM Primers

Epoxy and DTM primer is simple to use and apply. First, check the manufacturer’s Technical Data Sheet (TDS). The product TDS will give you the proper safety information, mix ratios and the expected drying times for ambient temperatures and force drying. Once it is mixed, you can brush or roll it on to large surfaces or, use a dauber or acid brush to apply it to small areas and flanges. It’s that easy because there is no airborne overspray and minimal masking. You could use a spray gun, but that tends to make the process more complicated. There are some two-component aerosol-can versions on the market, but airborne overspray and masking can complicate the use of these products as well.

The more you embrace the fact that Epoxy and DTM primers can bolster your corrosion prevention and adhesion efforts, the fewer excuses you will find not to make them part of your repair procedures. Toyota’s Collision Repair Information Bulletin #186 Titled ‘Corrosion Prevention for Collision Repair’ provides more in-depth information about the use of Epoxy and DTM Primers.  

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