Answer
To remove mill scale, use 80 or 120, then refine with 180, 220, and 320 before primer.
Why it happens
Mill scale is a hard surface layer and needs aggressive enough abrasion before finer prep.
Recommended grit
Use 80 or 120 for removal, then 180, 220, and 320 for paint prep.
Wet or dry
Dry.
Success check
The steel is uniformly dull with no shiny scale patches.
What to do
- Clean oil from the steel.
- Use 80 or 120 to cut through mill scale.
- Refine with 180 and 220.
- Use 320 before primer if a finer prep is needed.
Avoid: Do not apply primer over shiny remaining mill scale.