Using rusty effects powder
Rusty Powder is a fine-grained ferro-powder that can be used to create genuine rust effects.
Mix with Powertex liquid and apply to your chosen object by palette-knife or brush. Spray with a water/vinegar-mixture - and repeat at will until the desired effect is achieved. Let dry at least 24 hrs before further handling.
More texture can be created by mixing Rusty Powder with molding paste, throw in some Powertex 3D Sand and spritz with vinegar water.
Powertex Rusty Powder - Powertex.co.uk
Rusty Powder Available In Store
Powertex Liquid Ivory With Bister or Rusty Effects - Available In Store