Industrial Painting: Coating Systems and Corrosion Protection Explained
Knowledge · Surfaces & painting
Why is painting a system and not a coat?
Because each layer has its own job. The primer bonds to the steel and carries the active corrosion protection, often with zinc phosphate pigments. Optional intermediate coats build film thickness and block moisture. The top coat provides colour, gloss and UV and chemical resistance. Typical in machine building are two-coat systems of 2K epoxy primer and 2K PU top coat; total film thicknesses range from about 80 µm indoors to 240 µm and more for aggressive environments.
What do corrosivity categories C2 to C5 mean?
ISO 12944 classifies environments and derives coating systems from them. For practice this means: tell the coater the service environment and you get the right system instead of a guess.
| Category | Typical environment | Guide total film thickness |
|---|---|---|
| C2 | heated halls, dry interiors | approx. 80–120 µm |
| C3 | production rooms with humidity, urban atmosphere | approx. 120–160 µm |
| C4 | chemical plants, industrial outdoor use | approx. 160–240 µm |
| C5 | aggressive industrial and coastal atmosphere | from approx. 240 µm, special systems |
Why does preparation decide durability?
Because no paint adheres better than its substrate allows. Mill scale, rust and weld spatter must come off before the first coat. The steelwork standard is blasting to Sa 2½ to ISO 8501-1: nearly pure metallic surface with a defined roughness for the primer to key into. Grease-free surfaces, broken edges and dressed weld transitions belong to it, because wet paint pulls back from sharp edges and film thickness collapses there. A large share of all coating failures traces back to preparation, not to the paint.
What does the buyer gain when the manufacturer paints in-house?
Three things: no transport between weld shop and paint shop with its packaging and schedule risks; a painter who knows the assembly and masks fits, threads and functional faces properly; and single-point responsibility from blasting through film thickness measurement to final inspection. A practical enquiry tip: note RAL colour, gloss level, service environment and faces to keep paint-free directly on the drawing; that eliminates the most common queries.
Frequently asked questions
What film thickness does a machine coating need?
For machines in heated halls (C2), usually 80 to 120 µm from primer and top coat. Outdoor or chemical environments raise coat count and thickness; ISO 12944 and the coating datasheet govern.
What does Sa 2½ mean?
A surface preparation grade to ISO 8501-1: blast-cleaned steel, nearly pure metal with only light shadows permissible. It is the standard prerequisite for high-performance corrosion protection.
Wet paint or powder coating?
Wet paint is flexible in part size, colour and repair and therefore standard in machine and plant building, especially for large parts that fit no curing oven. Powder shines on small and medium series parts.
How is coating quality verified?
Film thickness measurement to ISO 2808, visual inspection for runs, craters and masking errors, and adhesion tests on request; a report documents the values per part.