To fully appreciate why surface prep is so critical to the success of repairs made with adhesive, it helps to pause and consider what is going on at a microscopic scale. Any foreign particles on a substrate, such as grease or dust, will physically prevent the adhesive from coming into contact with the intended surface. Similarly, having insufficient surface area, usually in the form of an overly-smooth substrate, will reduce points of contact available between the adhesive and the substrate, compromising the strength of the bond.
A few simple steps will result in a substrate that is in optimal condition for an adhesive to bond to:
- Clean with soap and water to remove any water soluble contaminates, dirt/mud, and general grime.
- Clean with a water-based or solvent-based cleaner designed to remove greases, oils, and silicones that soap and water did not remove. Make sure that all surfaces are dry before proceeding to the next preparation step. Leave time for solvents to evaporate from surfaces, especially plastics, as water or solvents can be absorbed into the plastic and adversely affect a repair.
- Abrade the surface and apply a surface modifier, if needed. Scuffing the substrate with a scratch-pad, sanding with a variety of grit ranges, or grinding are all techniques that can create more surface area. An abraded surface typically has double or triple the surface area compared to the original flat, smooth surface.
Sanding and grinding generate heat that can be detrimental to plastic surfaces, so use the finest grit possible. Go slowly to reduce heating and to avoid off-gassing or the release of plasticizers, which can make the surface slippery, as well as to avoid burning the plastic.
Avoid Read-Through: Read-through that happens 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 overall repair area being visible. If a too-coarse sanding media is used in surface preparation, read-through takes place 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 – such as when technicians rush through or try to cut a process short.
Learn proper preparation techniques: Understanding how to properly prepare the adhesive to be applied is just as important as the surface prep itself. Even when taking care to follow every step for correct surface prep, adhesive bond strength can be compromised if the adhesive itself is not prepared properly. Be sure to always follow the adhesive application instructions and technical data sheets for mixing two-component adhesives, leveling the cartridge and purging the mixer before beginning a repair procedure. Otherwise, collision repair shops run the risk of the bond failing.
A properly prepared surface will help ensure a successful repair with a bond that will last to protect the integrity and safety of the vehicle.
There are many resources for the body shop technician to learn about proper surface preparation. In addition to the hands-on training offered by many product manufacturers and distributors, including our distributor-partner Saint-Gobain Refinish Solutions Group, always check original equipment manufacturer (OEM) guidelines for surface preparation requirements and adhesive recommendations.
Surface preparation training is also available from vocational and technical schools, as well as other organizations offering aftermarket repair programs such as I-CAR (the Inter-Industry Conference on Auto Collision Repair).
It is always best to follow OEM procedures, which include details on the ideal surface prep for the substrate being repaired. If a specific adhesive is approved, then the surface should be prepared per that adhesive manufacturer’s standard operating procedure, unless the OEM has determined otherwise.
To learn more about proper surface preparation, take a look at these I-CAR training courses and this ABRN article.
I-CAR Surface Preparation Training
The following I-CAR courses cover surface preparation as part of the curriculum:
- Aluminum Intensive Vehicle Repairs (AL101) – Understanding the characteristics and best practices for working with aluminum in structural applications.
- Adhesive Bonding (ADH01) – Identifying adhesive bonding materials, including how to prepare parts for bonding with one- and two-part adhesives.
- Welded and Adhesively Bonded Panel Replacement (EXT02) – Explains exterior panel construction and replacement, including preparation procedures for door skin replacement, removal and installation of doors skins, and installation of door hinges.
- Squeeze-Type Resistance Spot Welding (WCS04) – Learn about welding equipment and power sources, how to make resistance spot welds, and how to prepare mating flanges for weld bonding.