How to choose shot size and hardness

Before choosing the abrasive, establish what you are using it for. There are three different purposes, and they ask for different things.

Surface cleaning — removing oxides, foundry sand, burrs. The most frequent case.

Cutting — applied for example to granite blocks, with special mixes of hard shot.

Etching — done to increase surface adhesion before coating. A classic example: preparing a bathtub before enamelling.

Size

For a good surface you need a mix of large and small shot. Not a single size.

Shot too large — cleans locally, but leaves peening marks on the surface.

Shot too fine — you do not reach the cleanliness grade you want.

The practical rule: the finest shot that still removes the residue. Not larger “to be safe”. Large shot hits harder, but covers fewer impact points for the same mass of abrasive.

Names differ between suppliers, but they map onto each other. The equivalence table, for shot (S) and grit (G):

Shot	Grit	Diameter	SAE
S780	G10	2.00 mm	SAE 10
S660	G12	1.70 mm	SAE 12
S550	G14	1.40 mm	SAE 14
S460	G16	1.18 mm	SAE 16
S390	G18	1.00 mm	SAE 18
S330	—	0.85 mm	SAE 20
S280	G25	0.71 mm	SAE 25
S230	—	0.60 mm	SAE 30
S170	G40	0.42 mm	SAE 40
S110	G50	0.30 mm	SAE 50
S70	G80	0.18 mm	SAE 80

The number in the name is the diameter in thousandths of an inch, times a thousand. S330 means 0.033 in, that is 0.85 mm. Useful when comparing quotes that use different notations.

Hardness

The principle is a single one: the abrasive must be harder than the surface being blasted.

If it is softer, it deforms instead of working the part. You consume abrasive without getting a result.

If it is much harder than needed, you get the result, but you pay more and wear the machine liners faster.

Impact energy

Shot is propelled with a certain kinetic energy, which depends on mass and velocity. Mass depends on diameter and density.

The consequence that matters: between close diameters, the energy per particle differs several times over. Which is why an apparently small change in size visibly changes the result.

Not everything that leaves the wheel arrives usefully on the part. Part of the energy is consumed inside the system — on impact with liners, in conveying, in separation.

Who decides, in the end

A good abrasive in a badly set machine does not give a good result. Nor does the reverse.

The right choice is made jointly: abrasive size, wheel settings and machine condition. The shot supplier and the machine supplier should be answering the same question, not two different ones.

Related reading

• Which blasting abrasive should you use? A guide by type
• How much abrasive you actually consume in blasting
• How clean is the surface: grades SA1, SA2, SA2.5 and SA3