Humidity control in blasting and painting

The dew point is the temperature at which the water vapour in the air begins to condense. The practical rule in blasting and painting is clear: the steel surface must be kept at least 3 °C above the dew point. Below that margin, moisture condenses on the steel and, on a freshly blasted surface, that film of water triggers rusting immediately.

Condensation is not just a cosmetic problem. The rust that forms between the steel and the paint breaks the adhesion that should exist between the two surfaces, and moisture also affects the curing and drying of the coating. The result is premature failure: blistering, delamination and corrosion under the film.
That is why the main paint manufacturers require a controlled-humidity environment and do not guarantee the performance of their coatings if applied outside those conditions.

There are three ways to keep moisture from condensing on the surface, with different effectiveness and cost:

• Heating the steel: keeping the part above the dew point. It works on small parts or in continuous blasting of plates and profiles, but becomes very costly over large surfaces (a blast room, an oil tank, the inside of a ship).
• Heating the entire work area: only about 50% efficient, impractical in extreme climates and uncomfortable for the operator.
• Dehumidifying the environment (recommended for large areas): it reduces the air's humidity, lowers the dew-point temperature, prevents condensation on the surface and reduces the degree of corrosion on the steel.

A controlled-humidity environment changes the economics of the job:

• It frees the operation from the weather: work can start early in the morning and even continue on rainy days.
• It keeps the blasted surface free of corrosion for 1 to 2 weeks, allowing the whole part to be blasted and then painted in a single application, after removing the dust.
• It keeps the abrasive (steel shot) dry and rust-free inside the blast pot, where overnight moisture would oxidise it: a key point in setting up the equipment.
• It speeds drying after water-based preparation —waterjetting/hydroblasting or wet blasting— and also serves to dry concrete before coating.

Humidity control is handled with equipment sized for the working climate: there are lines for warm, temperate and extreme temperatures, and versions with a pre-heater to operate efficiently at low ambient temperature. The ranges, capacities (in m³/h) and model selection are covered on the dehumidifiers page.

Humidity control is not an accessory to the process but a condition for guaranteeing the result. Keeping the surface above the dew point prevents premature rusting, ensures coating adhesion and durability, protects the abrasive and frees the job from weather dependence. On large blasting and painting projects, it is the difference between a coating that lasts and one that fails prematurely.