WHEELS 'N CUSTOMS TECHNICAL GUIDE: BODYWORK, SILL INSTALLATION & RUST PREVENTION

Optically lowering a car’s sides with side skirt diffusers (extensions) and emphasising the roof or boot line with a spoiler cap are among the most popular upgrades in automotive styling. Brands such as Maxton Design, Rieger Tuning and Ingo Noak engineer these aerodynamic trims with utmost precision from high-grade ABS plastic. They give the vehicle a lower, wider and more aggressive appearance that blends seamlessly with your alloy wheels and lowering kit.

In practice, however, crucial technical questions arise during installation: "How do you fit side skirt extensions without the metal sill starting to rust from the inside?", "Why is 3M tape alone not sufficient for a storm-proof installation?" and "How do you professionally degrease paintwork with 3M 4298UV primer for a rock-solid bond of a boot spoiler?" In this technical guide, the bodywork specialists at Wheels 'N Customs explain step by step how to install body parts in accordance with German TÜV NORD safety guidelines, with a 100% guarantee of a rust- and paint-damage-free result.

Side skirt extensions are mounted to the underside of the existing sills. Depending on the car brand and factory trim (for example BMW M Sport, Audi S line or VW GTI), the lower sill consists of a plastic add-on sill or directly of the load-bearing steel chassis section. As soon as you drill into load-bearing steel, bare metal is created that is directly exposed to moisture, road salt and mud.

While driving on the motorway, side skirt extensions are subjected to a constant swirl of airflow, spray from rainwater and stone chips kicked up by the front wheels. In accordance with the official TÜV NORD guidelines and Article 11 of our Terms and Conditions, a side skirt must never be fitted using double-sided tape alone:

• Steel or Aluminium Blind Rivets (Pop Rivets): In our workshop in Nijkerk we preferably fit side skirts with heavy-duty multigrip pop rivets with an extra-wide flange. The rivet pulls the ABS trim tight against the sill and can never work loose due to vibration.
• Stainless-Steel (A2/A4) Body Bolts: In locations where the inside of the sill is accessible via inspection plugs, we use stainless-steel M6 bolts with wide body washers and self-locking nuts. Stainless steel prevents galvanic corrosion between the fasteners and the bodywork.
• Elastic MS Polymer Body Sealant: Along the full contact edge between the skirt and the sill we apply a thin bead of vibration-damping body sealant. The sealant acts as an acoustic damper, prevents dirt and sand from chafing between the parts, and distributes the downward suction force evenly.
• Loss at High Speeds: For parts that are attached only with tape or simple self-tapping screws and come loose at high speed, Wheels 'N Customs does not provide an installation warranty in accordance with the terms. Always choose proper mechanical retention.

Unlike side skirts, small tailgate spoilers and spoiler extensions (spoiler caps) are almost always installed with high-quality double-sided 3M VHB acrylic foam tape. This means there is no need to drill into the tailgate, keeping the paintwork 100% intact. The durability of this bond, however, depends entirely on the chemical pre-treatment of the car paint:

Whether self-installing or having the parts fitted in our workshop, strict guidelines apply to prevent disappointment and warranty disputes:

• Mandatory pre-fitment check (dry-fit): Always hold the side skirts or spoilers against the car ‘dry’ on the lift before any drilling, bonding or sealing. Check that the length is correct from wheel arch to wheel arch and that doors swing open freely without touching the skirt. In accordance with Articles 11 and 13 of our General Terms and Conditions (the processing prohibition), any right to return or claim lapses as soon as holes have been drilled, protective films have been removed or adhesive/sealant has been used.
• Protective film on Gloss Black items: Maxton Design parts in high-gloss black come with a robust protective film. Leave this film fully on the part during measuring and drilling to prevent tool marks. Only once the skirt or spoiler is finally installed should you remove the film in one smooth motion.
• Fixing during curing: After fitting spoilers with 3M tape or bodywork adhesive/sealant, the part must remain fixed under light mechanical pressure for at least 12 to 24 hours with body tape (low-tack masking tape). Avoid high-speed driving and the use of pressure washers or car washes within 48 hours of installation.
• Mind the jacking points: When measuring side skirt extensions, ensure the original jacking pads and lift support points under the car remain clear and accessible. This prevents your skirts from being damaged when the vehicle is later lifted on a ramp at a garage for a tyre change or service.