Your only mistake was you were served by an idiot. The muesli look I think means you had contamination bleeding into your paint. Did you have the correct sealer undercoat? You don't need thinners on the roller, it's for the tipping brush, you just throw the rollers away regulary really your better off with 3 coats of the right undercoat which is cheaper and easier to sand, then the topcoat looks a lot better. You can roll two pack but they are special 4 inch high density rollers that you buy in packs of 20, after you roll the paint on you go over it lightly with a 4 inch brush in a process called tipping. If you contact GDB services at Kilkenny they're a family owned business that mainly wholesale's marine paint to the trade but they will sell retail and they know paint. GDB have the rollers, the right thinners (thats extremely important because your tipping brush will have a bit of thinners on it )and sand paper I've also rolled and tipped International Brightside a single pack with quite reasonable results