Vallejo Chipping Medium is a water-based medium to replicate broken paint, scratches and rust spots.

How to use Vallejo Chipping Medium for rust spots:

1. Apply a good primer to your miniature layer (for example Vallejo Surface Primer)
2. Apply the "rust" color (for example Vallejo Model Air Rust)
3. Optionally bring a darker rust color for more realism later
4. Then apply the Chipping Medium to the places where you later want to see the rust spots.
5. Let this dry well!
6. Now paint the figure the way you normally would.
7. Scratching can begin. Use, for example, an old brush that you cut very short so you only have 2-3 mm hair left.
8. Only scratch the "new" paint on the spots where it is desired, so that the underlying "rust" colors become visible.
9. Highlight to finish it off completely, the edges of the rust spots.

For an extensive tutorial look at this blog item.
New waterbased Medium to create flaked paint, scratches, and rust chips. Chipping This medium can be applied with airbrush or brush to create chipping effects with waterbased acrylic colors.