Body filler over OEM paint can provide higher quality repairs, protection

LKQ training manager describes recommended procedures

Atlanta—Body filler technology has come a long way since the original plastic filler, Bondo, was introduced in 1955 to replace toxic lead-based body solder. With today’s higher-quality and more flexible lightweight fillers, application processes can help a collision shop deliver a better repair with greater corrosion protection, said Alan Craighead, North America technical and training manager for the PB&E Division of LKQ Corp.

Not that long ago, it was common practice to prep the repair area by grinding to bare steel with sandpaper as coarse as 36-grit, and featheredging paint back from the repair area with a finer grit, such as 180, to prevent any filler from extending into paint. If a technician didn’t follow those processes, a ringed area known as a “bull’s eye” or “repair mapping” would often show after the repair was complete.

“Those scratches in the paint were not really scratches; they were plowed edges,” Craighead said. “And those plowed edges, if you think of a field that you plow, it looks like it’s only six to eight inches deep, but if you stick your hand in the ground, it goes down another six to eight inches. So basically, those scratches would collapse and they would crumble underneath because they were not really cuts; they were tearing and ripping the paint. So putting a body filler over that type of preparation — even a quality one — is going to fail.”

To prepare the bare metal area, Craighead said, 80-grit should be used. And by using a slightly finer grit to featheredge the paint (240 instead of 180) and applying higher-quality fillers, the latest of which are epoxy-based, filler can be overlapped onto OEM paint. Craighead said he has recommended that procedure to LKQ/Keystone customers for about six years, but recognizes that more technicians need to be aware of it. Because modern fillers sand more easily using finer grits, it’s an easier adjustment for technicians to adapt to the relatively new procedure, he said.


Undercured OEM finishes can cause repair-mapping issues

In addition to providing increased corrosion protection and minimizing the size of the repair area, spreading filler into feathered OEM paint is one way to address the issues that undercured OEM finishes can present, Craighead said. The issue can often be detected by a shiny area where either the basecoat meets the primer or the clear meets the basecoat, he said.

“I’m not saying it’s completely not cured, but it is still slightly reversible and will shrink on you,” he said. “When you apply any kind of urethane surfacer or epoxy over it, it will eventually shrink down to that, because that is an uncured system.”

In addition to the techniques mentioned above, technicians can infrared-dry the primer, or for shops using UV primers, the UV primer technology will hold down this undercured finish and prevent any ringing from showing up later.

“There are differences in opinion from different body filler manufacturers,” Craighead said. “But my field experience has shown if it’s scratched correctly with the finer grits and you don’t sand through the edge of the clear, you eliminate that ringing effect and you have a quality repair.”

One paint company’s testing showed that adhesion of quality body filler is even better to a correctly sanded OEM finish than to bare metal, he said, owing in part to the heat created during the body filler’s curing process. And in a deformation test, it had the same flexibility as filler applied to bare metal, he added.

“So that’s where it’s evolved,” Craighead said. “Will the industry ever accept it? I think body filler manufacturers and paint manufacturers need to do more testing. Because if you can leave that e-coat and OEM finish intact without breaking it, you’re going to have great corrosion resistance there. And the testing has already shown that quality finishing body fillers stick better to the paint than metal, so that could be the trend the industry goes toward.”


Specialized paintless dent repairs may become more popular

Craighead said he thinks that relatively new types of paintless dent removal tools used by collision repair shops in Europe will become more popular.

Instead of sanding to bare metal so that metal studs can be used, the modular system uses high-strength adhesive and specialized plastic tabs with a repair bridge to remove deep dents without harming the paint.

“So now you have the OEM finish — which is a great quality finish — that’s not been broken, and they’re simply sanding with 240 or finer and putting the body filler right over the area once they’ve finished it out. Obviously, if you have any cracked paint, you’d need to feather that out. But if you’re doing any bridge-type repair, it’s a quick repair but is very gentle on the metal and the paint finish.”

Parts & People

Parts & People is published monthly by Automotive Counseling and Publishing Company, Inc., a Colorado corporation, P.O. Box 18731 Denver, CO 80203, 303-765-4664. President-Lance Buchner. Founded by Lance Buchner and Dave Lucia.