In workshop practice, in order to be able to carry out a consistent and objective evaluation of the scope of the damage, a degree of rust on the scale of 1 to 5 is determined by the DIN 53 210 standard.
The main criterion here is the extent to which rust exists under the paint structure. It is determined in millimeters (mm).
Underlying rust grade: R1 < 1 mm
Corrosion starting with up to 1 mm of rust underlying (in the form of a spot or a line).
The damage can be rectified by cleaning the defective location and mechanically removing the underlying rust. For a small extent, apply a primer using a brush and allow it to dry. Touch-up the location with a paint pen or provide a new paint coating.
Underlying rust grade R2 < 1 - 2 mm
Advanced corrosion with up to 2 mm underlying rust.
Rectifying the damage:
Underlying rust grade R3 < 2 - 4 mm
More advanced corrosion with up to 4 mm underlying rust. The damage must be rectified in the same way as for R2. A permanent cure of this type of damage pattern is still just possible
Underlying rust grade R4 < 4 - 5 mm
Notably advanced corrosion with up to 5 mm underlying rust. The damage must be rectified in the same way as for R2. If it is found that for whole areas this is only possible with a great deal of work, or is not possible at all, then a new component must be used.
Underlying rust grade R5 > 5 mm
Extreme corrosion, with more than 5mm underlying rust (panels, flanges or load-bearing components partially rusted or rusted through).
Such damage can no longer be repaired because in many cases the constructional strength of the component can no longer be produced. The risk in making a repair is too great. Install a new component and paint it.