One way of removing paint or mill scale is by shot, grit or bead blasting where I 17 objects are fired at the surface in order to chip away the paint and any corrosion before re-painting.

Shot is normally recycled and often uses small harbor steel balls. Grit is normally used once and uses irregular shaped copper slag or chilled iron. Bead blasting uses irregular shaped plastic beads. The size of these particles and the velocity at which they are fired in the air stream determines how quickly the paint is removed and the profile of the remaining substrate, irregular shaped particles producing a coarser profile than regular shapes.

Blast cleaned substrates cannot be solvent wiped because of the metal profile, so must be blown clean and primed or suitably protected as soon as possible within 4 hours indoors.

Where coarse blasting is used and a large metal profile is produced. It is important to apply sufficient primer to cover the peaks of the profile. This often means filling the valleys of the profile, otherwise poor resistance of profile peaks can occur.

Commonly used grit are 012, G17 and G24 with mesh size .30-.71mm, .42-.85mm and .60-1.oomm respectively, and shot S120, S170 and S240 with similar respective mesh sizes.

Primers suitable for blast cleaned Steel include phenolic-modified type of etch primers. Zinc phosphate in polyamide cured epoxy and metallic zinc in T polyamide cured epoxy resin systems.

Etch and wash primers contain zinc tetroxychromate dispersed in an alcoholic solution of polyvinylbutyral resin. The etching properties are contributed by phosphoric acid, which is generally added immediately before use. The dry films are very thin and should be overcoated quickly.