Did you receive a GrillSymbol or Petteri Wiimaa Cor-Ten product but it doesn’t look like on the picture? Don’t worry, here we advise you how to process weathering steel to look like on the pictures.

What is Weathering Steel?

Weathering steels are Cor-Ten and S355JOWP. Both are high-strength structural steels with exceptional weather resistance. The chemical composition of the steel allows for the creation of a patina layer that covers the product without altering the mechanical characteristics of the steel. This natural rust layer not only protects steel products from the weather, but also provides a unique eye-catching finish. Due to its longevity, weathering steel is more environmentally friendly than conventional steel. Weatherproof steel products can be outside all year round.

What to do?

When brand new, Cor-Ten and S355JOWP are gray in color like regular carbon steel. Due to weathering, it initially turns reddish-brown and becomes even darker over time. The reddish-brown patina is a natural protective layer similar to rust that prevents further corrosion and extends the life of the product. When weathering steel gets wet, it becomes thicker, unlike regular carbon steel, which is made thinner by rust.

Unpacking the product: to avoid excess moisture, the products must be unpacked immediately after receiving them, so that any moisture accumulated in the packaging can dry out and prevent uneven rusting.

Surface Seasoning: The normal rust process or a quick version.

Normal process: leave you Cor-Ten steel product outside, rain and moisture will cause a layer of patina or rust. The metal surfaces have been pre-cleaned with a solvent – this means that the surface is ready for the formation of rust. Depending on the conditions, the first rust appears after 1-3 weeks. The natural rusting process of Cor-Ten steel takes about 3-5 months, depending on weather conditions and the climate of the location. The product can be sprayed with water to speed up the process.

The quick process (NB! Only at your own risk) means spraying the salt solution onto metal surfaces. This must be repeated 5-6 times. The suspension should be dilute, about 1 teaspoon of salt per liter of water. Dissolve the salt in warm water and spray on metal surfaces. No other treatment is needed, the rain will wash away the salt over time. After being sprayed with salt water, the product must not remain in dry conditions such as indoors or under the roof, but must be rinsed repeatedly either by rain or sprinkling.

Weather-resistant steel and staining: When a layer of rust forms, the product emits rust in the first year. If you place the product on a wooden deck, stains may appear on the surface underneath the product. Some types of stone absorb stains, and rust can give stones a brown color that can be difficult to remove. After the first year, the weatherproof steel has reached its layer of rust and should not stain anymore. The manufacturer is not responsible for the elimination of features (e.g. possible rust stains) due to the specifics of the product’s material. To minimize staining, flower pots and containers with bottoms stand on M6-M10 bolt heads with a height of 6-10 mm, depending on the model. Such a solution creates an air gap between the product and the (smooth) surface, extending the life of the product and saving the surface.

If you want to use GrillSymbol or Petteri Wiimaa products indoors or slow down the rusting process, it is recommended to use Owatrol 732SK rust oil. Owatrol darkens rusty surfaces slightly while providing a light shine.

Flowerpots and planter boxes that have soil inside all year round: for drainage, we recommend putting a layer of light gravel ⌀10-20 mm thick at the bottom of the box about 10 cm. The manufacturer does not recommend installing flower pots directly on soil or grass, because excessive moisture and direct contact with the surface can cause an uneven layer of rust.