Sticking with adhesives

July 8, 2015
Create the best urethane bonds possible using our step-by-step guide of the latest adhesive solutions.

Competition drives every industry. The collision repair industry has its fair share of manufacturers offering products that promise to either top or replace those from their competitors.

One of the more interesting competitions has been between bonding solutions such as rivets or welding and urethanes. Each has applications where one is by far the better choice (sometimes the only choice), along with cases where the two compete directly. In the relatively new frontier of aluminum repairs (gaining in significance with each passing year), rivets and MIG welding have become indispensible.

Don't count adhesives out. Through steady improvements in chemistry, they too offer repair options for aluminum, along with other metals and perhaps most importantly, plastics.. Refer to the following instructions for some of the latest adhesive solutions.

Points on plastics
Broken, torn or ripped bumper covers are the usual candidates for plastic adhesive repair. Considering the extensive use of plastics through most vehicles, many other areas also may need repaired. The following steps, supplied by the Urethane Supply Company, outline the basic steps for using most adhesives regardless of the damaged part.

Step 1: Identification. Working with plastics almost always begins with the identification of the plastic type. Plastics react differently with adhesives depending on their makeup. Shops must first identify the type of plastic in the part, which typically is indicated by a code stamped into the part. If you are unable to locate this, refer to the manufacturer information. From there, locate the adhesive recommended for the repair.

Note here that some manufacturers offer adhesives that work despite the plastic or substrate type, saving shops some significant time.

Step 2: Cleaning. Clean both sides of the plastic with warm water and soap. Next, use a plastic cleaner to remove any wax or remaining contaminates.

Step 3. Clamping. Fix the front side of the part with aluminum tape or clamps to hold it together while the adhesive on the backside (to be applied later) cures.

(Courtesy of Hannibal Body and Paint) Adhesives have improved the point where they can handle a range of plastic and metal repairs that once would have called for a replacement part. (Courtesy of Urethane Supply Company) Always use dedicated cleaning products to remove contaminants that might otherwise weaken an adhesive bond.

Step 4: Sanding. Use 50 grit sand paper or coarser  to abrade the backside of the repair area. Then, use a DA sander with 80 grit paper on the surrounding area on the front. If the opposite side of the part also needs sanded for cosmetic reasons, you'll need to create a v-groove. The extensive sanding is necessary since "heavy grooving" of plastic helps maximize the strength of the bond. Use dry, clean compressed air to blow away any remaining sanding particles.

Note: Some plastics (for example, PP, TEO and TPO or PP) also will need an adhesion promoter following the sanding.

Step 5: Backing and reinforcement. Select a reinforcing method for the back of the part. For flat areas, it may easier to cut a backing plate out of scrap material. For contoured areas, drywall tape or fiberglass cloth will work better.

Prepare the reinforcement by cutting 1-3 pieces of glass cloth to cover a damage area that is 2-4 in. wide. If you're also using a backing plate, make sure it extends at least 2 in. beyond the damage areas in all directions. You'll also need to rough grind the side of the backing plate that will be applied to the surface.

Step 6: Mix and apply the adhesive. Mix according to package directions. Apply a generous amount on the back of the part. When using a backing plate, press the plate firmly into the adhesive, so a small amount of adhesive will squeeze out from the edges. If you're using fiberglass cloth, lay the cloth into the adhesive and wet the fibers with a saturation roller. Apply more adhesive over the area. Embed another layer of fiberglass cloth, if necessary.

For repairs using a v-groove, apply the adhesive into v-groove with the body spreader. Slightly overfill the v-groove so it can be sanded flush when finished.

Step 7. Final moves. Once the adhesive on the back has cured, peel the aluminum tape off the front.

For repairs utilizing a v-groove, use a die grinder and/or a coarse sanding disc to create a 1-2 in. wide groove. Sandscratch the inside of the v-groove coarsely. Round off any sharp edges and feather the paint back with 80 grit paper using a DA sander.

When the adhesive on the front side is fully cured, sand with 80 grit in a DA sander. To finish, gradually progress to finer grits prepare for painting.

Metal patching
Adhesives often can create bonds on metals just as strong as those built with welding. They can be particular useful when patching panels when welding isn't an option. The following steps, from the Eastwood Company, detail patch panel work on both steel and aluminum.

Note: When finishing, panel adhesives can withstand temperatures up to 250 degrees F. Also, any welding performed near must be done at least 4 in. away from the cured panel.   

Step 1. Preparation. Use a DA sander and 180 grit paper to remove all paint. At least 1-2 in. of corrosion free metal should surround the repair.

Prepare cut lines at least 2 in. from the damage or near panel edges or parting lines.

Feather paint 2-3 in. around the cut lines for later blending. Use a drill or pneumatic nibbler to cut out the scribe line (a hand nibbler or shear can be used, though will more difficult).

Step 2. Create a flange. Use a panel flanger to neatly and accurately flange around the opening of the cutout section. Adjust the vise grip to produce a flange deep enough for a flush repair. Check the flange with a piece of scrap metal of the same gauge to make sure the flange is correct and that both pieces create a level surface.

Lay out the dimensions of the cutout area on a piece of cardboard. Measuring from the outside of the flange, cut to the exact shape and transfer to your patch panel (which must be the same gauge as the original metal). Check for correct fit, using a straight edge. Adjust the patch panel by carefully trimming it to fit level and flush all around flange area.

(Courtesy of Car Finishers) - Adhesives require extensive sanding of the repair area to build the bond. (Courtesy of Lord Fusor) Always check with vendors for new adhesive and adhesive kits, which can cut repair times and costs.

Step 3. Lay out the rivet pattern on the patch panel. Keep the rivet holes centered in the overlap area. Start the rivet layout at a corner to assure it lays flat when completed.

Depending on the size of the patch, space rivets about 3/4-1 in. apart. For smaller repairs (for example, if the patch is only 5 in. long by 3 in. high). use 3/4 in. spacing. If the patch is larger, use the 1” spacing.

Note: Use only aluminum rivets and patch panels on aluminum and steel rivets and panels on steel.

Step 4. Put the patch panel in place. After laying out an accurate rivet pattern, clamp the patch panel using edge clamps (and c-clamps if needed). Check for flush (level) positioning of the panels using a straight edge. Drill the rivet holes with a 1/8 in. bit, starting in the corner and using blind holders in the drilled holes to secure the panel. Once all the holes are drilled, remove the clamps and patch panels and de-burr all holes.

Step 5. Dimpling. Use dimpling pliers to countersink (dimple) all holes in both the patch panel and the attachment area. Once the holes are dimpled, install patch panel with the blind holders and test fit. The panel should be flush with the surrounding area and should follow any contours on the original panel.

If the panel doesn't fit flush, the original panel holes may not be correctly dimpled or the edges could be deformed. Fix either by straightening the edges or increasing the depth of the dimpling tool. If necessary, reform the lower or original panel dimples.

Step 6. Clean and prepare the adhesive. Remove the blind holders and clean the flanged area on both panels with a pre-painting product. Wipe the panels completely dry with a clean cloth or paper towels. Prepare the adhesive using manufacturer directions.

Note that adhesives have strict working times (usually, 30 minutes or less). If a mixing tip is used, they are typically not reusable.

Step 7. Application. Apply the adhesive on the original panel along the center of the rivet line in a 1/4 in. bead. Use blind holders to attach the patch panel in every hole, keeping the panel flush using a straight edge.

Some adhesive will be squeezed out along the flange area. Once the rivets are set, it can be smoothed with a body filler spreader.

Step 8. Riveting. Remove the blind holders one at a time, replacing them with a rivet. (Be sure to soak the holders in lacquer thinner to prevent the adhesive from setting up or the holders will be inoperable.)

To keep the panel flush, begin riveting at a corner or radius. Working evenly from the starting point, continue installing rivets until all of them are set. Use a spreader to smooth out the adhesive, which should protect the entire flange area from rust on both sides of repair.

Step 9. Following the adhesive manufacturer's directions, give the adhesive the recommended time to set up before sanding the repair area level. In most cases, the area can then be sanded as it would be if a conventional filler had been applied. The repair area will need additional time to cure before shops can apply any fillers, primers or topcoats, depending on manufacturer recommendations.

One final note, don't hesitate to speak with your vendors about the latest adhesives they offer. Adhesives come in an increasing array of solutions for nearly every bonding need. They could be saving your shop time and money at this moment.