Oliver, I get almost mould like spotting on a plastic cowl. It comes clean with Autosol Marine Shine polish (made in Germany), but it comes back. I`ll be watching the responses too.
Yours sound like the salt, a deliquescent water absorbing substance, drew on the plastic. Polish, and maybe Armor-all as a useful detailing product.... I think they came from when we towed it for a day and a half continuous and the sun dried the salt on...
White vinegar is always a product I try for stains and water deposits. Just don't let it dry on the surface. Wet surface, apply vinegar, let it "work" for a few minutes, rub with microfiber or soft brush and rinse.
Does anyone know of a compound or cleaner that can take water spots of a plastic cowling? I've tried a sort of cleaners at no avail.