Rust grade (A to D)
The condition of steel before preparation, graded A to D in ISO 8501-1: A still carries intact mill scale, B has rusted with scale flaking, C is rusted with scale gone and D is pitted. The starting grade limits what preparation can achieve and what the finished surface will look like.
The rust grade is an input, not a result. It describes what the steel was before anyone touched it, and it constrains the outcome: pitted grade D steel blasted to a near-white grade still has pits, and those pits hold contaminants and abrasive that a visual inspection will not find.
Specifying a preparation grade without recording the starting rust grade removes the information needed to judge whether the result was achievable at all.
Sources and references
- StandardISO 8501-1:2007 — Preparation of steel substrates before application of paints and related products — Visual assessment of surface cleanliness — Part 1International Organization for Standardization · accessed August 29, 2026
Where this is covered in depth
- SSPC-SP1 to SP11: Surface Preparation Degrees ComparisonA complete guide comparing SSPC surface preparation standards from SP1 to SP11, including NACE and ISO 8501 equivalents for industrial coatings.
- Flash rust: the window between blasting and coatingWhy a surface prepared to SP 10 can fail its own specification hours later, and how to specify a holding time that survives a schedule.