How to protect sills from salt and de-icing chemicals in winter

Side sills are one of the first places where the body starts to rust. They sit low, salty slush flies at them from under the wheels, and inside the sill there is a closed cavity where moisture with salt can sit for weeks. We explain why sills rot faster than the rest of the body, how to wash them in winter, and what really helps—and what does not.

Contents

• In short
• Which sills we mean: exterior and door sills
• Why sills rot faster
• What to do before winter
• How to wash sills in winter
• Door sills: scuff plates and mats
• What not to do

In short

• Sills often rot from the inside—rust appears in the closed cavity earlier than a blister shows up on the paint.
• Drain holes must stay open—water leaves the sill through them; a clogged hole keeps salt inside.
• Wash the underside along with the sills regularly, not just the body “until it shines”.
• Door sills are damaged not so much by salt as by sand and de-icers from shoes—here, scuff plates and EVA mats do the job.

Which sills we mean: exterior and door sills

The word “sills” refers to two different things, and you protect them differently:

Why sills rot faster

Anti-icing mixes are chlorides: regular technical salt, as well as calcium and magnesium chlorides. The latter absorb moisture from the air, so residue from them stays wet for a long time and keeps eating away at the metal even on a dry road. The wheels throw this slurry straight onto the sills, the lower edge of the doors, and the arches.

A sill is a closed box section. Moisture with salt gets inside through technological openings and seams, and it should leave through drain holes at the bottom. If they are clogged with dirt, a salty solution can stand inside the sill for years, and rust goes from the inside out: when a blister appears on the paint, the metal under it is already thin. The second reason is chips: sand and gravel from under the wheels knock paint off the lower part of the sill, and salt gets straight onto bare metal.

What to do before winter

• Clear the drain holes at the bottom of the sills and doors—using thin wire or a plastic stick, without enlarging the hole.
• Renew the anti-corrosion treatment for cavities. A wax-based preservative is applied inside the sill through the factory openings using a long nozzle: it displaces water and keeps salt away from the metal. The openings are not sealed.
• Cover up chips. Touch up small chips on the sills before the морозs, and the lower part of the sill can be coated with an anti-gravel coating.
• Install mud flaps if you do not have them: they reduce the amount of dirt and salt that flies from under the wheels onto the sills. Mud flaps in the catalog include vehicle-specific ones for factory mounting points, as well as universal ones.

How to wash sills in winter

You can and should wash your car in winter—the main thing is to do it correctly (more details in the article can you wash a car in winter). For sills, here is what matters:

• Wash them together with the underbody and wheel arches—salt left underneath keeps working even after the body is gleaming.
• Regularly: roughly once every one to two weeks, and more often after driving on heavily salted roads.
• Water first, then shampoo: salt crystals and sand scratch the paint, so do not put a sponge on a dirty sill.
• Rinse the lower edge of the doors and the door openings—salty slush accumulates there.
• Wipe the seals and the door sills dry so the doors do not freeze, and water does not stand in the opening.

Door sills: scuff plates and mats

The door sill is rubbed by soles every day, and in winter sand and de-icers from shoes remain on it. This wears the clear coat on the sill down to the primer, and then salt reaches the metal. The simplest protection is scuff plates: they stick to the sill and take the abrasion. For commercial vehicles there are EuroCap ABS scuff plates for specific models (Mercedes Vito, Fiat Ducato, Volkswagen Caddy and others), which are attached with double-sided tape.

In vans with a high step into the cabin, EVA step mats for sills work well: snow, sand, and water stay in the cells of the mat rather than on the metal, and the mat is easy to shake out. Before sticking on a scuff plate, the sill must be washed, degreased, and dried in warmth—tape will not adhere to cold, damp metal.

For exterior sills, besides washing and treatment, there are side steps: they partially shield the body sill from spray and stones, although they do not protect the cavity itself from moisture.

What not to do

• Do not seal over or plug the drain holes—the sill will start rotting faster because water will have no way out.
• Do not knock ice off the sills with a scraper or hammer—this is how chips appear.
• Do not pour boiling water on frozen doors and sills—temperature shock harms the paint, and the water will freeze again.
• Do not postpone washing until spring—salt works all winter, and by spring you already have to treat rust.

More generally about protecting the body from de-icers—in a separate article about salt and de-icers.

We deliver with Nova Poshta across Ukraine; orders from 15,000 ₴—free with our own transport.

How it works

Clean the drains

Find the holes on the underside of the sills and doors and clean them out with a thin wire or a plastic stick, without enlarging the hole.

Treat the cavities

Apply a wax-based preservative inside the sill through the factory openings using a long nozzle. Do not seal the holes after treatment.

Touch up the chips

Touch up chips on the sills before frost; if necessary, cover the lower part of the sill with an anti-gravel coating.

Protect the door sills

Wash, degrease, and dry the door sills, then install covers or place mats on the steps.

Wash regularly

In winter, wash the car together with the underbody, wheel arches, and sills, and wipe the door openings dry.