Mix the powder with Powertex or Easy Sturcture and spread with pallet knife or brush. Spray a mix of water and vinegar on the wet surface and repeat as much as necessary. Let dry 24h or more.
You obtain a rust paste by mixing Easy Structure with Rusty Powder and a dash of vinegar.
Mixing Powertex with 3DSand, Rusty Powder and a dash of vinegar, results in a coarser paste and a rough rust effect.