Underseal

Underseal is a thick tar-like coating which is sprayed onto the underside of cars to protect against stone chips.

Most cars come with underseal already applied at the factory, but additional underseal is sometimes applied to off-road vehicles for extra protection.

There are several companies in the UK specializing in this service. However, this is not a service New Again Ltd provides.

What it means

Underseal is an umbrella term for underbody protection systems. Traditional bitumen/asphalt products form a thick, black film; modern options include rubberised or polyurethane stone-chip coatings, wax-oil type creeping waxes for cavities and seams, and transparent underbody wax for easier inspection. Correct surface prep is critical: degrease, remove loose rust, convert or prime remaining corrosion where appropriate, dry thoroughly, then apply in controlled thickness with attention to seams, flanges and drain paths.

Why it matters

• Corrosion control: seals against water, salt and grit at vulnerable seams and exposed metal.
• Impact resistance: rubberised/PU systems absorb stone strikes in arches and on leading edges.
• Noise reduction: thicker elastomeric films can damp road roar and drumming panels.
• Serviceability: transparent or well-documented underseals make future inspection and repairs easier.

Where you’ll see it

Older vehicles and restorations, daily drivers in coastal or high-salt regions, 4x4s, vans and classics. Also used after welding or subframe repairs to reinstate protection.

Context

Car Paint Protection; Rust prevention; Underbody protection; Bodyshop refinish

Types at a glance

• Bitumen/asphalt underseal: very thick, black, hides blemishes but can crack with age and trap moisture if applied over rust.
• Rubberised/PU stone-chip: flexible, impact-resistant, paintable; suited to arches and sills after proper priming.
• Wax/oil cavity protection: thin, creeping; ideal inside box sections, seams and doors. Needs periodic renewal.
• Transparent underbody wax: lets you see what’s happening underneath and monitor corrosion.

How it’s done (at a glance)

• Inspection & masking: map corrosion, protect exhaust, brakes, rubber mounts and sensors.
• Prep: pressure-wash and degrease; mechanically remove loose rust; dry thoroughly; treat remaining rust (converter) or prime with epoxy where specified.
• Application: observe temperature/humidity, apply recommended film build in multiple light passes, keep drains and breathers clear.
• Cures & checks: allow solvent flash and cure; recheck for misses, runs and blocked holes; document with photos.
• Maintenance: annual inspection, wash the underside, touch-in chips and re-wax cavities as needed.

Common mistakes

• Coating over active rust, damp or road dirt – traps moisture and accelerates corrosion.
• Blocking drain holes, jacking points or breathers – causes water pooling and future rot.
• Over-spraying exhausts, brakes, rubber bushes and belts – creates smells, fire risk or degraded components.
• Applying bitumen too thick – leads to sagging, cracking and water ingress behind the film.
• Hiding structural corrosion – heavy black underseal can mask MOT-critical issues and complicate repairs.
• Skipping documentation – without photos and product notes, future technicians cannot assess what’s under there.

Written by Danny Argent. Last updated 11/11/2025 15:07