How to underseal a car properly (step by step)

Undersealing a car properly is not one job, it is six. Skip any of them and the coating traps damp against the steel and makes the rust worse rather than better. This is the order we work in at Rayco.

1. Wash and degrease

The underside is pressure washed and degreased so the coating has clean steel to key into. Oil film, road grime and old flaking coatings all have to come off first.

2. Strip back loose material

Flaking underseal, scale and loose rust are scraped and wire-brushed back. Rust hides under old coatings, so anything that lifts has to go.

3. Inspect and weld where needed

With the underside clean, weak steel shows up. Perforated rails, outriggers, sills and floor sections are cut out and new steel MIG welded in before any coating goes near them.

4. Treat the remaining corrosion

Sound but corroded areas are treated so surface rust is stabilised rather than sealed in.

5. Cavity wax the box sections

Chassis rails, sills and crossmembers rot from the inside out. A wand-applied cavity wax is injected into the box sections through existing access holes — this is the step DIY jobs almost always miss.

6. Coat the exposed underbody

Two coats of heavy-duty underbody coating go onto the exposed steel, indoors, with brake components, exhaust and sensors masked off.

Can you do it yourself?

You can brush a coating onto a driveway, but without a ramp you cannot clean the steel properly, you cannot weld what needs welding, and you cannot get wax into the cavities. That is why brushed DIY jobs typically fail within a couple of Irish winters.

Want it done properly? WhatsApp underside photos to 087 919 2193 for a quote before you travel.