Powder Coating of steel structures

Powder paint is a dry powder of resin, pigments and additives. The steps:

• preparation: degreasing, removing rust, scale and old coatings, abrasive cleaning, phosphating if needed, then drying (moisture under the film causes defects);
• application: the powder takes an electric charge and settles on the earthed product;
• curing: in the oven the powder melts and chemically cross-links into a continuous film;
• cooling and inspection.

The powder is usually thermosetting: the hardened film cannot be re-melted, and the cure schedule is set by the powder manufacturer.

• a continuous, uniform film;
• the product must withstand heating: rubber seals, plastic and lubricants are removed;
• powder settles poorly in deep corners and cavities (the Faraday cage effect), so tubes are usually not coated inside: zinc protects them;
• sharp edges get a thinner film, so they are rounded;
• on bare steel, rust creeps under a damaged film, while in a duplex system the zinc holds it back.

Colour is set from the RAL catalogue. RAL is a scale of shades, not a paint recipe: appearance depends on gloss, texture and substrate, so where an exact colour match matters, a colour sample is agreed (a RAL fan deck or the customer’s reference). Shade, gloss and texture availability is confirmed when we price the order.

Outdoors, polyester or polyurethane powders are used: epoxy fades in sunlight and turns chalky, so it is normally kept indoors.

A duplex system is hot-dip galvanising plus a powder coating. According to the American Galvanizers Association, depending on the environment such a combination can last 1.5 to 2.3 times the sum of the service lives of each coating alone. Read more: Durability of hot-dip galvanising.

Our own galvanising bath is 13,000 × 1,600 × 3,200 mm and there is no minimum order: Hot-dip galvanising. For the powder to hold on zinc:

• agree the coating before galvanising: passivation against white rust impairs adhesion, so it is left out where needed;
• grind off runs, drips, icicles and hard inclusions of zinc: they show through the film;
• on older zinc, remove dirt, grease and white rust; fresh zinc, once drips are ground off, only needs rinsing and drying;
• roughen the surface by light sweep blasting with a soft abrasive at an angle, or by phosphating: ordinary sand blasting removes too much zinc;
• pre-heat the product so that moisture and air leave the zinc pores, otherwise the film gets pinholes and blisters (outgassing);
• coat straight after preparation, before oxides form on the zinc again.

EN 15773, the European standard for duplex systems, lists what the customer, galvaniser and coater must agree.

• no grease, rust, scale, paint residue, slag or weld spatter on the surface: powder reproduces every irregularity;
• edges are rounded and welds ground smooth;
• no closed cavities, and holes for air and liquid in hollow members: liquid from the pre-treatment baths trapped in an enclosed space boils in the oven and spoils the film;
• lap joints and narrow gaps are avoided: liquid from a gap later runs out as streaks. Small overlaps are sealed with a continuous weld around the contour, and on products to be galvanised larger ones are vented as in Designing for hot-dip galvanising;
• threads, bolt holes, earthing points and contact surfaces are marked on the drawing; they must be masked with plugs or heat-resistant tape, and we agree the masking with you.

• a drawing or sketch with dimensions, weight and quantity;
• surface condition: bare steel, galvanised or already painted; whether duplex is needed;
• the RAL colour, preferred gloss and texture, and a colour sample if required;
• service conditions: outdoors or indoors, close to the sea, roads treated with de-icing salt or an industrial zone;
• areas not to be coated, the desired lead time, and the city and hand-over method.

We check the product’s dimensions against the oven’s working chamber (6 m long, 1 m wide, 1.8 m high), and confirm batch size and lead time after reviewing the drawings.