Let me give a brief description of some of Turner’s methods, quote from Joyce Townshend:
He scratched out darks with a sharp point, his thumbnail, or the end of a brush, or stippled over them with a hard, dry brush, removed color with blotting paper, rubbed out areas of watercolor wash with stale bread, gave texture to areas of color with his thumb or fingers or washed them out with clean water. Highlights were also added in gouache…. Some watercolors of the 1790s have areas reserved by stopping out, the application of glue size to a patch to prevent other washes covering it, until finally the resist was washed away to leave a well-defined patch of paper. Careful control of very wet washes gave hard edges to areas of color, while soft edges were achieved by sponging off paint, or by moving the pigment around with a wet brush.
I’d recommend purchasing this book published by the Tate Gallery. A great value for the money. How to Paint like Turner is a corny title, but the book actually has a sound overview of his methods, dry brush, spatter, wet-in-wet, charging, gradated washes, stopping out… many of the effects Turner used. Of course, a lot of other watercolorists use them but none like Turner. The obvious thing they lack is his vision, which informed every mark he made.