What a rust inhibitor does in a car wash

What the step is

In the Fast Lane Luxury Wash, the rust inhibitor comes right after the under-car spray. In general, car wash rust inhibitors are light products sprayed onto the underside that leave a thin film behind. The film is meant to push water away from metal surfaces and slow the reaction that turns steel into rust.

We are not going to tell you it lasts a set number of weeks or protects a specific part. How long any wash-applied film stays on depends on how much slush and rain the car drives through afterwards, and in January that can be a lot.

Why the order matters

An inhibitor sprayed over a crusty underside would seal salt in instead of keeping it out. That is why it goes on after the under-car spray has rinsed the floor, the subframe and the wheel arches. The sequence is clean first, then protect.

It also explains why the step is most useful on a car that is washed regularly. If the underside is rinsed every week or two, the inhibitor is going onto metal that does not have a season of salt sitting on it.

What it cannot do

A rust inhibitor in a wash is a light maintenance step, and it is easy to expect too much from it.

• It does not remove rust that is already there or turn it back into sound metal.
• It is not the same as a full rust-proofing treatment, where a shop sprays oil into door cavities, rocker panels and frame rails.
• It does not cover chipped paint on the body, which needs touch-up paint.
• It does not last indefinitely. Road spray and later washes wear it away.

When it makes the most sense

The inhibitor is most useful from the first salting through to the spring thaw. That is when the underside is wet and salty most days, and anything that slows corrosion between washes helps.

It is also worth considering on a car you plan to keep a long time, on an older vehicle where some surface rust has started on the subframe or exhaust, and before a stretch when you know you will not get to a wash for a while, like a busy month or a trip.

Habits that work alongside it

The inhibitor works best as one part of a routine. Rinse the underside regularly all winter. Keep the drain holes at the bottom of the doors and hatch clear, since water trapped inside a door rusts it from the inside out. Touch up stone chips on the hood and lower doors each fall. If you get an annual oil rust-proofing done, keep doing it; the inhibitor fills the gap between those treatments rather than replacing them.

Finally, look under the car when it is on a hoist for a tire swap. A rust inhibitor gives you more time between problems, but you still want to see what is happening under there.