Shot blasting throws small steel shot or grit at a part at high speed to clean it. It removes rust, mill scale, sand and old paint, and leaves a rough, even surface that paint and coatings grip well. Shot peening is a controlled version used to strengthen springs and gears.
Quick facts
- Media
- Steel shot, steel grit, garnet, glass beads, aluminium oxide
- Machines
- Tumble, hanger, table and blast-room types
- Common standard
- Sa 2½ (near-white metal) before painting
- Surface profile
- About 40–100 µm for coatings
- Used on
- Castings, forgings, fabrications, plates, structures
- Usually priced
- Per kg, per square metre or per piece
What it is used for
- Cleaning sand and scale from castings and forgings
- Removing rust and mill scale before painting or powder coating
- Giving an even matt finish on machined or fabricated parts
- Removing old paint
- Shot peening: adding compressive stress to improve fatigue life of springs, gears and shafts
Cleanliness grades
| Grade | Meaning |
|---|---|
| Sa 1 | Light blast cleaning |
| Sa 2 | Thorough blast cleaning |
| Sa 2½ | Very thorough, near-white metal; usual before industrial paint |
| Sa 3 | White metal, completely clean |
Grades from ISO 8501-1. Coating suppliers usually state the grade they need.
What to tell the blaster
- Part material, size and weight
- Finish grade (e.g. Sa 2½) and surface profile, if a coating needs it
- Areas to mask: threads, machined fits, bores
- Whether it will be painted soon after (bare blasted steel rusts quickly)
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Questions
- What is the difference between shot blasting and sand blasting?
- Both blast abrasive at the surface. Shot blasting uses steel shot or grit, usually in a closed machine where the media is reused; sand blasting traditionally used sand, which is now avoided for health reasons and replaced by grit or garnet.
- What does Sa 2.5 mean?
- A blast-cleaning grade from ISO 8501-1: near-white metal, with only slight stains left. It is the usual requirement before industrial painting and powder coating.
- How soon should blasted steel be painted?
- As soon as possible, ideally within a few hours, because the clean steel starts to rust quickly, especially in humid weather.
The figures on this page are typical values for guidance, written by Diemeco from standard engineering practice. They are not a specification: confirm grades and properties with the relevant standard or the supplier's mill test certificate, and tolerances with your workshop. Last checked 15 October 2026.