Repair that cracked front spoiler in seven easy steps

By Tom Suddard
Sep 3, 2022 | repair, Shop Work, body | Posted in Shop Work , Restoration & Renovation | From the Feb. 2010 issue | Never miss an article

Photography by Tom Suddard unless otherwise credited

Some of the most iconic cars in history, from Fords to Ferraris, are particularly striking when viewed head-on. The nose of a car makes a powerful first impression: Whether good or bad, it becomes etched in your memory.

We have a 1987 BMW 325is in our fleet, and lately it hasn’t been making the best impression. These cars have a low-hanging front spoiler, so it only takes is one good speed bump or curb to splinter the unit into tiny pieces. Unfortunately, our car has suffered this fate.

The easy fix would be to just replace the cracked piece with a new one, but those can cost hundreds of dollars—if one can be found at all. We decided to repair our spoiler for a fraction of the cost. Dex Manders, a body tech at The Body Werks in Holly Hill, Fla., showed us the proper method.

Whether working with paint or plastic, preparation is everything. The more work put into the prep, the better the finished product will look.

Step 1: Scuff and Tape

First, we scuffed the front of the bumper and covered it with high-temperature duct tape. The tape simply holds the cracked pieces together during the repair. After scuffing the cracked areas from behind, we blew off the resulting dust. 

Step 2: Strip and Clean

We used a special wax and grease remover—in this case PPG Acryli-Clean—to clean the area. We then sprayed 3M Automix Polyolefin Adhesion Promoter all around the crack to make sure the epoxy would stick. After that, we covered the backside of the repair area with mesh tape; much like the rebar that’s placed in concrete, this mesh tape adds reinforcement to our spoiler.

Step 3: Add Epoxy

Plastic epoxy—we used 3M Automix EZ Sand—can now be generously applied over all of the mesh tape. Borrowing the applicator gun will save a healthy chunk of change on this step. We left the epoxy to dry for about 45 minutes.

Step 4: Gotta Groove

Now we could work on the front of the bumper, and the procedure here is a bit different. We used a rotary tool—a Dremel works well—to cut a very deep, 1/2-inch-wide, V-shaped groove along the entire length of the crack. This allows the adhesive to bond to itself on the backside of the bumper, making the repair very strong. It also prevents the repair from sporting a raised ridge, so the end result is seamless.

Step 5: Sand and Clean

After cutting the groove, we sanded down the surrounding area to bare plastic. Then we again cleaned the bumper with our wax and grease remover before applying the adhesion promoter.

Step 6: Fill the Crack

The front was now ready for the epoxy, which we applied generously over the crack. Once the epoxy was dry, we sanded.

Step 7: Smooth it Out

Next came the last step before painting. We applied a thin layer of Bondo over the repaired area to smooth any remaining imperfections.

All Done

After sanding down the Bondo, we finished the repair with a quick coat of paint. Now that the spoiler is repaired, it looks like new—and it didn’t cost a fortune to fix. Our BMW can finally make a good first impression.

Photography Credit: Per Schroeder

Join Free Join our community to easily find more repair, Shop Work and body articles.
View comments on the GRM forums
Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
8/25/20 9:27 a.m.

What material was that spoiler made of exactly? I know this article is a decade old, but wouldn't the repair technique vary with the type of plastic?

Tom Suddard
Tom Suddard GRM+ Memberand Director of Marketing & Digital Assets
8/25/20 9:32 a.m.

I can't remember exactly what E30 spoilers are made of, but yes–it's important to match your materials to the plastic. Any body supply store will be able to give you the right stuff if you tell them what you're working on, though.

z31maniac MegaDork
8/25/20 10:29 a.m.

It's also different on the 88+ cars. There isn't that "air dam" hanging below the front bumper made of plastic. 

The later valances are metal then with an "i" or "is" spoiler attached to the metal valance below. The "is" spoiler being deeper with a raise middle profile section.

The really hip thing to do is get an '86-87, then use the later 88+ parts. You have to do some trimming on the rear bumper and bumper caps for fitment. But then you get the larger rear wheel wells of the earlier cars, with the much better looking plastic bumpers of the late cars. 

11/19/21 3:20 p.m.

Please don't use Bondo on flexible materials, it's virtually guaranteed to crack as soon as the surface flexs. There are an entire suite of flexible bodywork products you need to use when working on urethane bumpers. Bondo might do a little better on plastic, but it can still crack if the bumper is twisted enough.

Similarly, when working on bumpers, every product needs to be designed to flex, from the filler to the primer and even paint. You can easily buy flexible black paint for bumpers in a can, but any paint supply store can mix you any color you want with the required flexible additive included.

Sorry, I worked at a bumper repair shop for a number of years, and there is nothing worse then trying to repair a bumper that was previously fixed or painted with regular products meant for metal. You have to sand the whole thing down back to the urethane or it will just crack again.

05CAR New Reader
9/4/22 12:03 a.m.

In reply to Keith Tanner :indeed, and as I recall the products to fix for the na miata front bumper are completely different from the rear bumper, right?


kb58 SuperDork
9/4/22 11:54 a.m.

I read the thread title as "The repair that caused the bumper to crack."

friedgreencorrado UltimaDork
9/4/22 12:04 p.m.

I swear, if someone ever writes a book about restoring 'modern' automobiles, half of it will be about how to repair plastic.

Randy_Forbes New Reader
9/5/22 9:04 a.m.

Plus nowadays, we have more trick tools like the hot stapler* (I frickin' LOVE mine) and plastic welders that include a variety of filler materials.

If there was one (1) thing I'd change about the article, it would be to use the DX330 (wax & grease remover) more liberally; use it to clean the area BEFORE you ever touch sandpaper to the area being repaired.

* If you haven't discovered this tool yet, and there's ANY plastic on your car, you owe it to yourself to check it out (prices can range from <$100 to >$300, so shop around for what suits your needs).

The brand/model  I have has since been superseded with a cordless version (@ 2X the price) but the money saved on this one (1) taillight assembly repair would've paid for tool (working only on BMW M Coupes/Rdstrs, it's undoubtedly saved my customers thousands of dollars by repairing plastic parts versus replacing__hell, in just glovebox mounting tab repairs__let alone underbody ducting & panels).

Our Preferred Partners