Here is the grading system in use today
courtesy of the BCCA and Beer Can World

Grade 1+ Virtually Perfect Can.  No noticeable imperfections. Only a few cans even come off the line in 1+ condition
Grade 1/1+ Extremely minor scratches or dings around the rim.  Lids may show some discoloration or tarnishing.  No humidity spots, no rust or dents.  No other imperfections.
Grade 1 Very minor scratches, marks, or humidity spots on the surface.  Cans are generally priced based on Grade 1 condition.
Grade 1- Very light fading of colors.  Very small humidity marks, scratches or dents on the surface.
Grade 2+ Noticeable spots, blotches, scratches, dents or rust spots.  Often applies to cans found indoors though occasionally a 2+ can can be found out-of-doors.
Grade 2 Has a clean label, but there may be rust pr other blemishes near the seam, the lid, or the bottom.  Colors may be irregular.
Grade 2- Label is somewhat clean but there is rust and other imperfections elsewhere on the can.
Grade 3 Multiple scratches, spotting, rusting, and dents.  Label is readable and the color is still decent.
Grade 4 A rusted can, but easily identified by brand, label.  Can will have fading, dents, rust, scratches, etc.
Grade 5 Very rusted and damaged.  Barely recognizable as to brand or label.

This grading chart courtesy of Beer Can World