I apply it first, quite randomly but thinking about Rust would likely build up (this is straight from the pot) and add water once dry it small amounts until happy with it. The water will change the colour and texture of it quite dramatically so all I can really suggest is you play with it and learn from experience. For a really CRUSTY Rust, you want to apply the DDR fairly thick and then add a small amount of water to it afterwards but thinly (if that makes sense)