Have you tried Artisan Salt Impasto yet?  The simplest way to get that aged layered sea-swept texture!  Just add a scoop of Salt Impasto to your paint and mix to a thick paste with the texture of lumpy cottage cheese. Apply to the surface using an old brush or chip brush.  We like to use a dabbing or pouncing action to add even more texture.  Allow to dry for 20 - 30 minutes and the run a dry brush lightly over the surface to slightly flatten high peaks.  Allow to dry and then apply your top contrasting colour.  (There is no need to add Impasto to this layer however you always can layer additional layers if you want to!) Allow to dry and then sand to reveal all that lovely crustiness and texture!