Structural Repair with Two-Part Adhesives

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Cover Courtesy Whitehorse Press
Within the realm of “how-to books,” there is precious little on repairing plastics and none with the comprehensive yet easy-to-follow scope of “How to Repair Plastic Bodywork.” Step-by-step procedures take you from preparing and reinforcing the site all the way through the refinishing process.

How to Repair Plastic Bodywork (Whitehorse Press, 2009) by Kurt Lammon contains easy-to-follow directions and clear illustrations applicable to any plastic repair — cars, motorcycles, trucks, ATVs or snowmobiles. Lammon explains how to assess the damage, identify the type of plastic and fix everything from a simple scratch to a major break. Learn about the types of two-part adhesives and the preliminary stages of structural repair in this excerpt taken from Chapter 4, “Repair Procedures.”

You can purchase this book from the Motorcycle Classics store: How to Repair Plastic Bodywork.

In body shops around the world, by far the most common products used in plastic repair are two-part adhesive materials. They get their name from the fact that they consist of two components, commonly called an A Side and B Side, or Resin and Hardener. The separate components are liquid or paste until mixed together in the proper proportions. When mixed, a chemical reaction begins that will result in the material becoming solidified. In other words, a two-part adhesive is, in itself, simply a thermoset plastic.

A specially formulated thermoset plastic, that is. The adhesive must have the proper balance of open time (working time), cure time, viscosity, strength, flexibility, adhesion, sandability, and durability to work effectively. That’s why there are many different types of two-parts available. There’s bound to be one designed for the job you need to do.

I’ll first briefly discuss the four generic types of two-part repair materials available. After that, I’ll discuss how to perform structural repairs with two-part adhesives.

Two-Part Adhesives: Four Types

The four generic types of two-part adhesives available for doing plastic repairs are epoxies, urethanes, acrylics, and polyesters. Most people who’ve had any experience with bodywork are familiar with polyester fillers. These are commonly called Bondo (which is a trademark of the Bondo/Mar Hyde Corporation), but are generically called body fillers. Polyester body filler is not usually recommended for plastic repair, but it does have its place.

More common for plastic repair are epoxy and urethane two-parts. Epoxies are easier to sand than urethane, but aren’t quite as strong. Urethanes are very strong and tough, but due to this characteristic, they are also hard to sand. One company, Lord Corporation, makes an acrylic-based two-part repair material that competes in this category.

Structural Repair with Two-Part Adhesives

Whether you use a rigid two-part epoxy or urethane adhesive, the preparation method and use of each is about the same. The basic rules of plastic repair surface preparation are as follows: clean the surface and abrade it. Cleaning will remove contaminates that may prevent the adhesive from sticking to the surface. Abrading the surface with sandpaper will increase the surface area for the adhesive to stick to. Both steps are critical to creation of a strong, reliable repair.

Abrade and V-Groove the Plastic

After you’ve cleaned the plastic, abrade the backside, or non-cosmetic side, of the plastic with coarse sandpaper, somewhere in the range of 36- to 80-grit. Very coarse sandpaper will put deep, heavy grooves in the surface and to some extent will allow the adhesive to lock in mechanically. Even if you sand with a 24- to 36-grit paper, it’s still a good idea to sand the area with 80-grit to further increase the total surface area available for the adhesive to stick to.

You may choose to V-groove into the substrate or grind it flat. If you cannot build up a reinforcing patch on the backside due to space limitations or appearance considerations, you will need to grind a V-groove into the plastic. If you can build up a reinforcing layer on the backside, you will achieve a stronger repair. In this case, it’s not necessary to V-groove into the backside, simply grind the plastic flat. This will allow you to have an even deeper V-groove on the frontside for added strength.

If the cosmetic appearance of the part is not important on either side of the plastic, it is best to V-groove halfway through on both sides and to build up a reinforcing layer on both sides to overlap the outer edges of the V-groove. Such construction can create a repair that is stronger than the original material.

If you decide to V-groove into the backside, it is helpful to first remove the plastic using a die grinder or Dremel tool. The depth of the V-groove should be about halfway through the plastic. The width of the V-groove depends on the amount of strength you require. A wider V-groove will expose more surface area to the adhesive, making for a stronger repair. A narrow V-groove will be sufficient for areas where you expect there will be no stress. In either case, your V-groove should range between 3/8 and 2 inches (10 to 50mm) wide.

Remove the Dust

Once you’ve got a good scratch on the surface and you’ve completely removed the shine, remove the sanding dust and chips. The best way to do this is to blow it off with a blow gun and a source of clean, dry, compressed air. Be careful not to recontaminate the surface by blowing oily or wet compressed air onto the part. If in doubt, just wipe off the sanding dust with a clean cloth. Wiping the dust off won’t remove as much dust as a blast of compressed air, but at least you don’t have to worry about contaminating the surface, which will cause even bigger problems.

At this stage, some plastic repair manufacturers recommend cleaning the plastic again with a plastic cleaning solution. This is okay to do as long as you allow enough time for the solvents to completely evaporate. The problem is that once you’ve sanded all these microscopic nooks and crannies in the surface of the plastic, reapplying the cleaning solvents will allow the solvent’s molecules to tuck themselves away deep within the sandscratches. These solvents may create adhesion problems and may cause the repair to bubble up when they try to evaporate.

Even though the plastic appears to be dry after wiping, the unevaporated solvents may still be hiding away within these sandscratches. Allowing 10 to 15 minutes of evaporation time, or heating the area slightly with a high-temp heat gun should be sufficient to ensure that all the cleaning solvent has evaporated.

In contrast, some plastic repair manufacturers recommend that you not apply a cleaning solvent after the sanding step in order to avoid this problem. As long as the plastic was cleaned prior to sanding and as long as clean sandpaper was used to abrade the surface, there should not have been any opportunity for the surface to become contaminated. (That is, as long as your hands are also clean! Don’t eat pizza while you’re repairing plastic!) Simply blow the surface dust-free.

This excerpt has been reprinted with permission from How to Repair Plastic Bodywork, by Kurt Lammon, published by Whitehorse Press, 2009. Buy this book from our store: How to Repair Plastic Bodywork.

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