Prioritize surface prep to ensure a successful repair

Whether you are working with metal or plastic substrates, preparing the surface properly before commencing with repair procedures is crucial to a successful outcome. Surface preparation involves more than just soap and water and a paper towel, although basic cleaning is important, too. Proper surface preparation comprises these steps – cleaning with a surfactant (soap), using a water-based or solvent-based cleaner, abrading, and applying a surface modifier, if needed. Once these steps have been completed, you can begin to repair a vehicle.

Surface preparation techniques
To start the surface preparation process, all surfaces and parts that will be repaired must first be cleaned with soap. Almost any soap will do – and many auto body collision shops usually have car-wash soap on hand, which is suitable for the cleaning procedure as long as it does not contain any wax.

In many instances, especially with panel-, weld- and metal-bonding, OEMs require removal of the e-coat. In situations where you are starting with a clean panel, cleaning with a solvent might not be necessary since the paint coating will be ground off during the preparation process. When working with vehicle parts from a car in use, any “dirty” parts definitely need a good wash-down. You will always need to water wash AND solvent wash every panel before repairs as some contaminants are water based and some are solvent based.  If you do not remove both types of contaminants they can be ground into the substrate while preparing the flanges.

Parts that are going to be repaired should be prepped with a solvent chemical such as mineral spirits, paint thinner, acetone, or isopropyl alcohol. Make sure that all surfaces are dry before proceeding to the next preparation step. Soap-and-water surfaces should be thoroughly dried. Leave time for solvents to evaporate from surfaces. Especially on plastic substrates, water or solvents can be absorbed into the plastic and adversely affect a repair, so a dry surface is essential. You always want to wipe any cleaning solvent off completely by wiping in one direction and turning your cloth often. You do not want to let cleaning solvents evaporate off the substrate as they will leave contaminants on the surface.

The next step in surface preparation involves abrading. Abrading can be accomplished by scuffing with a scratch-pad, sanding with a variety of grit ranges, or grinding. By abrading the surface, you are actually creating more surface area. An abraded surface typically has double or triple the surface area compared to the original flat, smooth surface. Many adhesive repair products need a “roughed-up” surface for good bonding results.

The abrasion process
An abraded surface optimizes the adhesive bond. While the surfaces will adhere without abrasion, the bond may not have the same level of strength as with an abraded surface and may be prone to failure. Adhesive bond strength is not a parameter that can be measured in the repair environment. You will only know that a bond did not work when the part falls off the vehicle after it has left the shop. A properly prepared surface will help to avoid this consequence.

When using sandpaper for abrading, use the finest grit possible to control heat output. Sanding generates heat during the abrasion process, and this heat can be detrimental to plastic surfaces. “Go slow” during the sanding process on plastics to avoid off-gassing or the release of plasticizers which can make the surface slippery and to avoid burning the plastic.

For abrading plastic surfaces, use sandpaper in the 80-to-120 grit range for the roughing work, and the 180-to-220 grit range for the finishing work. With metal substrates and for panel-, rivet- and weld-bonding surface preparation, 80-grit is the norm for bonded joints. Before doing any sanding procedures on aluminum, check with OEM specifications. Some OEMs require specific sandpaper for use on their aluminum vehicles and parts.       

Avoiding read-through
Read-through that occurs after repairs are completed can be due to several issues. There are two types of read-through: the actual repair showing through the surface and the repair area seen through the surface. If a too coarse sanding media is used in surface preparation it will show through on the surface when the paint finishing coat is applied. Using an excessive amount of primer will also show through on the surface.

In plastic repairs, read-through occurs when the adhesive is not fully cured. If the repair technician follows the repair procedures supplied with the product, there should not be any issue with the repair. Issues occur when technicians rush or short cut the process.

Adhesive cartridge preparation

As much as proper surface preparation is of prime importance, correctly preparing an adhesive cartridge is also an essential step that must not be overlooked. Always follow adhesive application instructions for leveling the cartridge and purging the adhesive before starting the repair procedure. Adhesives are usually sold as two-component systems in a one-to-one or two-to-one component package. One part is the A-side and one part is the B-side, or one part is the A-side and two parts are the B-side. In either case, the parts must be mixed together to achieve the proper ratio.

To level the cartridge after the cartridge has been placed in the applicator, extrude a small amount of the adhesive to make sure that parts A and B are dispensed from the cartridge. Before applying the adhesive, attach the mixing nozzle and extrude a mixer’s length bead of adhesive as waste to make sure the material in the nozzle is a homogenous mix. If the adhesive is not prepared properly, the bond will fail regardless of how well the surface was prepared.

Learning preparation techniques
There are many resources for the body shop technician to learn about proper surface preparation. Technical data sheets and user instructions supplied with repair products provide information on how to prepare surfaces before applying products. Many product distributors and manufacturer’s representatives offer hands-on training for proper surface preparation. Also, be sure to check OEM recommendations for surface preparation requirements on certain substrates (such as aluminum) or for specific replacement parts and vehicles.

The solvents and modifiers that are needed to provide a good bond should be sold as a system along with the adhesives. Consult with your distributor or manufacturer’s representative to make sure you have all the components needed to complete a successful bond.

Surface preparation training is also available from vocational and technical schools, and other organizations offering aftermarket repair programs such as I-CAR (the Inter-Industry Conference on Auto Collision Repair). I-CAR is an international, not-for-profit organization dedicated to providing the information, knowledge and skills required to perform complete, safe and quality repairs.          

The importance of proper surface preparation cannot be emphasized enough, according to Jason Bartanen, Director – Industry Technical Relations at I-CAR. “Without proper preparation, you are setting up for potential failure in the joint being repaired,” said Bartanen. “If you are not preparing the surface properly and do not have a good surface for a solid bond, you are compromising the safety of the vehicle.”

In the courses that I-CAR offers on vehicle repair, following product manufacturer or OEM recommendations for cleaning and preparation are highlighted as the first steps to completing a quality repair. “If you are going to apply an adhesive, whether by itself or in conjunction with rivet bonding or spot welding, a properly prepared surface is paramount to creating an effective joint,” Bartanen said. (See sidebar for more details on I-CAR courses covering surface preparation techniques.) 

Why it is important
Every repair procedure should begin with these steps: cleaning with soap-and-water, wiping with a water-based or solvent-based cleaner, using some form of abrasion, and applying a surface modifier, if needed. Once these steps have been completed, the surfaces to be repaired are ready for adhesive application – but don’t forget to properly prepare the adhesive and the cartridge. Every repair job should begin with proper surface preparation to avoid problems further along in the repair process and to ensure an enduring bond.