Preparing Structural Steel for Protective Coatings

New structural steel arrives covered in mill scale — a brittle blue-grey oxide layer formed during hot rolling. Paint applied over mill scale looks fine initially, but the scale eventually detaches and takes the coating with it.

Blasting to Sa 2½ removes mill scale, rust and any old coating entirely, and creates the angular anchor profile that primers grip. On a correctly prepared and coated structure, protective systems routinely deliver 15–25 years before first maintenance; on poorly prepared steel, failures appear within two or three.

Chloride contamination is the other common cause of premature failure, particularly on coastal and marine steelwork in Ireland. Salts left on the surface pull moisture through the coating and cause under-film corrosion and blistering, so washing as well as blasting is often required.

Timing matters just as much. Blasted steel begins to flash rust within hours in Irish humidity, so the primer should be applied the same day and before dew point conditions are reached.

Rayco blasts structural and fabricated steel in fabrication shops and on site nationwide, and coordinates directly with painting contractors so the coating goes on inside the correct window.