The repel needs to be applied directly to the fabric and not to dirt. This should also wash out any salt and sweat, and these will reduce the effect of any water repellent. Ideally this should also wash out any previous coatings.
Decided to give Diver Dave's stuff a good try after using Nikwax stuff since that and Paramo gear came out. I've just fully washed, dried and reproofed a Laksen Mouflon jacket and a traditional wool flat cap (like in the DD ad). I've also washed a couple of Tilley hats and a my old Seeland Keeper jacket to try it out on them. The Laksen jacket took a good 300ml, sprayed on with the jacket flat and worked into the outer material. I'll give some feedback next time it pi$$es down, it shouldn't take long light rain showers forecast for tomorrow here.