Preparation - Painting Steel

Mill Scale

One of the most common causes of paint failure on exposed exterior steel is painting over mill scale. New steel is covered with an oxide layer, that has a different expansion rate than the steel underneath, which will crack as the steel goes through expansion and contraction. Moisture enters the cracks, causing rusting between the steel and mill scale, causing the mill scale to delaminate, taking the paint with it. The only way to remove mill scale is by abrasive blasting.

Anchor Pattern

The sandblasting profile (anchor pattern) on steel should be about 25% of the thickness of the paint that will be applied and should not exceed the thickness of the primer. If the anchor pattern is 1.5 mils and the thickness of the primer is less than that, the peaks of the sandblasted steel will be above the primer. Flash rusting of the steel could occur before the next coat is applied.

Sandblast

The surface profile of a sandblast on steel is determined by the grit size and not the speed at which the blast hose is moved. The slower the blast hose is moved, the better the grade of cleaning, but the profile remains the same.

Rusty Surfaces

Painting over a rusty surface is seldom a good idea. Many coatings are available today that are recommended for application to a mildly rusted surface, but their performance is dramatically improved with better surface preparation. Paints don't usually penetrate through rust (especially rust scale) and paint performance deteriorates with the degree of rusting. It is poor painting economics to invest in a good paint system and put it over a rusty surface.