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Body Kit Installation | Weekend DIY, or Professional Shop?

Body kits are not always bolt-on, and may have minor imperfections. We clarify advantages of using a pro who can heat, reshape, shave, fill, and paint the pieces.
Body Kit Installation | Weekend DIY, or Professional Shop?

At CARiD, we understand how deeply the appeal of side skirts, rear diffusers, aero wings, air dams, custom replacement bumper covers, and other components included in body kits runs in the blood of automotive enthusiasts. We know, because we've added them on our own vehicles to enhance performance and to create uniquely impressive visual signatures.

We also feel it's important to point out that a lot of these parts are not easy to install. While the materials they're made from are highly durable, corrosion-resistant, and long-lasting, they typically come out of the box with minor imperfections that need correcting by experienced specialists. Whether you've purchased a body kit made of fiberglass, polyurethane, ABS plastic, or carbon fiber, we strongly recommend you take these parts to professionals that are experienced in body kit installation. In other words, they need to be familiar with the art of heating, re-shaping, shaving, filling, and painting these materials - because doing so is not the same as working with traditional metal body panels.

Installed Body Kit

While experts will tell you the "quality" of most aftermarket parts can vary, they also say that should not be a deterrent to purchasing what you really want. They also point out that consistency of smaller aftermarket manufacturers will never match the large scale perfection of major automakers, which are billion dollar companies with huge sums invested in sophisticated body panel manufacturing. It simply isn't going to happen. They admit any aftermarket manufacturer selling parts at real-world, non-"exotic" prices is going to have production runs that are excellent, good, and not-so-great.

Example Of Inconsistent Fitment Gap
Experts in body kit installation agree that while imperfections in aftermarket parts are common, they should not be a deterrent to purchasing what you really want. Surface irregularities, inconsistent fitment gaps, and minor cracks can all be remedied with ease - IF you know what you're doing.

In the spot-on words of one installer, "You're not guaranteed to get something perfect out of the box, that's why they say 'professional installation recommended' if you want it to look like a flat-out killer paint job. Someone who's experienced can take a $150 part and make it look like it's part of a $2,000 body kit. A pro knows how to take out all the waves, ripples, the imperfections, the thick & skinny areas, and fill pinholes in the material. What we do is make it all look perfect, and blend it into the car."

Body Kit Parts
When your body kit arrives, quickly inventory what's in the box before signing to ensure all parts listed on the packing slip are accounted for. Then look at them to ensure there isn't any major damage. Never assume everything is fine just because the outside of the box is fine.

To help you get a better idea of what to expect, we'll take a look at some of the approaches used by professional body kit installers when it comes to various materials. This isn't a how-to guide, but a look at specialized issues to help you realize that installing a body kit isn't for those without the knowledge, experience, and equipment.

Test Fit Items On Vehicle
Professional installers know it's essential to test-fit items on a vehicle before beginning any work, because parts cannot be returned once any alterations are made.

Our first recommendation is to open your box when it's delivered to you and inspect all parts before signing. Quickly inventory what's in the box to verify everything listed on the packing slip is accounted for. Then look at the items to insure there isn't any damage - never assume everything is fine just because the outside of the box appears to be in good shape.

As we mentioned previously, don't be alarmed if you notice some minor scuffs and discoloration on the parts - that's normal and will not affect the product's finished appearance. But if you see significant damage or are missing parts, you'll need to write "damage" or "shortage" on the delivery bill before you sign.

A professional installer will test-fit items on your vehicle before beginning any work, because they know parts cannot be returned after being altered in any way. Inconsistent gaps between factory OEM panels and aftermarket products is not uncommon, and the experts usually expect this. As we mentioned, body kit parts can be heated, trimmed, filled, and filed into shape with relative ease.

If a body kit includes larger components such as fenders, doors, and hoods, pros will take the trouble to assemble them on a vehicle before they even begin any mods. For example, adjustments to latching mechanisms and hinges can be made to ensure proper fit. Inconsistent panel gaps can be corrected using a number of other adjustment areas on the vehicle.

Spectacular Looking Part
Virtually all add-on aero parts typically require some amount of trimming, bonding, filling, and sanding before painting to ensure the finished result looks spectacular on your vehicle.

Checking fitment and panel gaps is an important first step, because it allows an installer to properly sand and remove excess material so things line up without rubbing. Once the fit is proper, items will be taken back off the car for paint preparation.

Professionals will look for any minor cracks, because they cannot be filled with body filler the way other imperfections can. Because filler will crack in seams where there's any tension, epoxy is used to cement the fractured area together permanently. Then, filler can be used to fill irregularities and smooth things over. However, it's important to note that filler does not work on all body kit materials.

Let's take a look at the specific installation approaches needed for different kinds of body kit materials.

ABS Plastic

Pros typically agree ABS plastic is one of the easier surfaces to work with. In decades past, painters had to take extra steps to identify which type of plastic they were dealing with if a material code stamp was missing. Fortunately, today's automotive plastics can be treated uniformly when it comes to paint.

However, particular caution is needed when sanding ABS plastic kits because the material itself absorbs more heat. When using power sanders, the necessary ridges created by sanding that are good for paint adhesion can soften and melt back to a smooth texture several minutes later. Excess sanding and heat can even cause "hairing" when plastic splits and melts into hair-like strands.

Surface Irregularities In ABS Plastic Part
Surface irregularities in ABS plastic parts sometimes occur, but are relatively easy for a pro to fix.
Plastic Adhesion Promoter
Plastic adhesion promoters open the pores of unprimered plastic material so that a new coating will adhere better.

After sanding, thorough cleaning with water and special detergents designed for plastic is the next step. For best painting results, an abrasive paste can be used to "texturize" the plastic as it's being cleaned. To ensure any wax and grease left on the body kit parts during production are completely removed, an installer may use a solvent-based or alcohol-based cleaner depending on recommendations from the part manufacturer.

Solvent-based (refined from petroleum) wax and grease removers are readily absorbed into raw plastic, while alcohol-based (distilled from grain) ones are not. One school of thought says alcohol cleaners flash on and off the surface too quickly to clean effectively, the other says this characteristic makes them a better cleaning agent. Once things have been cleaned, body filler can be applied over irregularities, then sanded smoothed and re-cleaned in a normal fashion.

Experts agree using plastic adhesion promoters that open the pores of plastic material so that a new coating will adhere better is a good idea, but that can be tricky. While adhesion promoters work exactly as intended over clean, sanded, bare plastic, they will not be effective on primered surfaces. Sometimes it's difficult to tell if ABS plastic parts already have primer on them because matte black plastic looks very similar to matte black primer. Pros say if sanding doesn't indicate the presence of primer, they'll check the instructions from the part manufacturer or flip the part over to compare the unprimed back side with the front.

Painted Part

Once a part is sanded and cleaned, plastic adhesion promoters work most effectively within a specific time window. They must be left on the plastic long enough to achieve maximum adhesion, but primer paint must be applied within a certain amount of time or the adhesion promoters start to dry and lose their effectiveness. Should any accidently get any on areas where body filler has been applied, it's quickly wiped off with a clean rag.

Some of the ABS plastic body kits we offer include the Vaero Body Kit, E&G Classics Effects Kit, Razzi Body Kit, and the RI Body Kit.


Polyurethane (also known as simply "urethane") is a popular material for body kits, and parts made of it typically come out of the box with less waviness than fiberglass parts. A polyurethane part can be "coaxed" into position during installation without fear of it cracking, and the end product will flex in response to minor impacts instead of breaking apart. Test-fitting polyurethane parts is important, because some areas of the part are purposely made with excess material to lower the risk of damage during transit. This may require shaving or filing down to create a correct fitment.

Peeled Body Kit
Unique care must be taken when polyurethane body kit parts are sanded, filled, and painted to avoid peeling - as shown in this extreme example.

While the finished surface of polyurethane parts is generally smoother and more even than fiberglass, don't expect perfection here either. Additionally, more specialized preparation is required in order for paint to adhere on polyurethane without peeling or flaking. Because it's softer, urethane cannot be rough sanded to remove unwanted coatings and grease from the factory the way fiberglass or ABS plastic can.

Instead, pros use specially-formulated urethane stripping solutions for proper initial cleanup, then perform a wet-sanding process with very fine sandpaper such as 600-grit to make the surface more adhesive to primer and paint.

They also know if there are any areas that need to be filled, traditional body filler will not work on polyurethane parts. As soon as any flexing occurs, the hard plastic filler will crack and fall off. Instead, they'll use flexible plastic filler to smooth over imperfections. Once that's been sanded smooth and cleaned, adhesion promoter can be used more or less same way as on ABS plastic parts.

When it comes to polyurethane body kits, we recommend 3D Carbon Body Kit, IVS Body Kit, Couture Body Kit, D2S Complete Body Kit, RK Sport Ground Effects Package, and the Xenon Body Kit among others.


Fiberglass is impregnated with small glass fibers for reinforcement and, as such, it's a particularly tricky material to work with. Experts say fiberglass parts almost always need more extensive work to get them ready for paint, and learning how to sand fiberglass requires a lot of patience and prep work.

Fiberglass Part
Fiberglass has its advantages, but expect aftermarket fiberglass parts to have some pinholes in the top surface which will need to be filled.

For example, fiberglass kit parts have a layer of epoxy or resin-based gel coat on the surface - the top most layer of which serves as a non-stick coating so parts pop free from their stamping molds during production. The gel coat also serves as the base surfacing layer on the fiberglass product, and this layer will often have waviness and small pin holes that need to be addressed before painting.

Next, fiberglass parts must be thoroughly cleaned with wax- and grease-removing agents to get all of the non-stick release coating off. It's critical to remove it all, because any that's left will contaminate the paint finish later. Next, the fiberglass is sanded initially with coarse sandpaper such as 120- or 180-grit. For large, flat surfaces, sandpaper mounted to a long sanding board makes the process easier. For tighter spots or areas with intricate curves, a malleable sanding block is most effective for reaching into nooks and crannies.

Pros say you should never sand all the way through the gel coat layer into the fiberglass itself. Doing so weakens the strength of the part and it creates holes in the fiberglass that cause paint to rupture later.

In fact, experienced installers know how to use the gel coat layer as a guide to determine when the right level of initial sanding has been achieved. Once the surface has lost its shine and become dull-looking to the eye, it's reached the optimum texture for primer and paint adhesion.

Without exception, every square millimeter of a fiberglass panel that will be painted must first be sanded and prepped in this fashion. Once a part is fully sanded, blemishes are easier to see and address.

Next, low spots are filled with fiberglass glazing putty, working it into low spots and sanding it away until uneven areas are flush with the rest of the surface. This step will require multiple stages of sanding and filling until the surface is true and free of defects such as pinholes, surface waviness, spider cracks, chips, or other problems.

The next step is to apply primer to the fiberglass, using a heavy primer coat with a high build primer suited for fiberglass (avoiding "etching" type primer that won't adhere well to fiberglass). The right type of primer doubles as a sprayable body filler, and it makes applying a uniformly thick layer over an entire piece much easier.

After the primer has set, all surfaces will be re-sanded again with finer sandpaper such as 180-, 220-, and 320-grit. Primer or paint is applied as desired, sanding between each application and using finer and finer-grit sandpaper. These last two steps are performed and re-performed until the installer is satisfied that the final result is free of waviness and defects and will reflect paint in a mirror-like way.

Some of the fiberglass kits we’ve got include the VIS Racing Body Kit, Duraflex Body Kit, Vertical Doors CA Super Coupes Body Kit (Corvettes only), and Buddy Club Body Kit.

Carbon Fiber

Because the whole point of carbon fiber is to show the look of the carbon weave fabric itself, parts made of it come with a protective surface coating of clear coat paint which can be buffed, re-finished and/or waxed should it be hazy or scratched. Carbon fiber kit parts are very strong, but also very rigid. This makes them easy to trim or cut, but impossible to re-shape. It's very difficult to break carbon fiber, but under enough stress things are more likely to break than bend.

Carbon Fiber Part
Cutting Carbon Fiber Part
Because of its high level of rigidity, carbon fiber can easily be cut or filed down - but not bent.

For this reason, the higher cost of carbon fiber typically includes a higher level of fitment that matches OEM dimensions, since it's not possible to modify these parts in the same fashion as more flexible materials. Should it be necessary to cut any functional holes in your part or trim the edges, your installer will sand edges along the cut lines, primer them, sand again, then apply black paint to go with the look.

Should you choose to paint over a carbon fiber part, every bit of the clearcoat needs to be sanded to a dull haze to provide a surface for paint adhesion. A pro knows the clearcoat layer must not be sanded away completely, only scuffed. Once this is done, the painting process would be similar to painting an ABS plastic part.

Body kits we offer with all or some carbon fiber components include the Carbon Creations Body Kit, Liberty Walk Body Kit, APR Performance Aerodynamic Kit, DefenderWorx C7 Corvette Revorix Style Body Kit, and Aero Function Body Kit (some parts may be urethane or other materials).

Body Kits Banner

To repeat some important points we made at the start: body kit installation is not for the typical home DIY'er. We strongly recommend professional installation. Make sure that the shop you choose to install your parts has experience installing body kits. Just because a shop is well-known for excellent auto body repair doesn't mean they have the uniquely special expertise and finesse required to adjust, sand, fill, and/or modify body kit components.

Naturally, consulting with whomever will be doing the work before you make a purchase is essential because regardless of the body kit material you choose, the end result will only be as good as the quality of the installation.

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