SPI Specs

The U.S. Society of the Plastics Industry (SPI) coded a series of steel tool finishes, noting as well the steps to achieve it. Following is the resulting list, that is regarded as a reference standard by most in the industry in the US. Also and for information, following is a cross reference correlation with an old finish classification no longer being used, but still being referred too.

A-1 (Grade #3 Diamond Buff)
A-2 (Grade #6 Diamond Buff)
A-3 (Grade #15 Diamond Buff)B-1 (600 Grit Paper)
B-2 (400 Grit Paper)
B-3 (320 Grit Paper)C-1 (600 Stone)
C-2 (400 Stone)
C-3 (320 Stone)

D-1 (Dry Blast Glass Bead #11)
D-2 (Dry Blast #240 Oxide)
D-3 (Dry Blast #24 Oxide) finishes.

Correlation of these finishes to the pre-1988 steel surface finish classification:

“High Polish” Finishes
A1 is comparable to #1
A2 is slightly finer than #2
A3 has more imperfections than #2

“Paper finishes”
B1 is finer than #3
B2 is slightly finer than #3
B3 is comparable to #3

“Stone finishes”
C1 is finer than #4
C2 is slightly finer than #4
C3 is comparable to #4

“Dry Blasted” finishes
D1 is finer than #5
D2 is comparable to #5
D3 is a little more coarse than #5

There are no current finishes that are comparable to the old #6 finish.