ZEP with or without decals? Before or After?

My experience having used a machine buffer and the Zeps is try to bring back the oxidized gel coat surface first with a buffer. If you can't then use the Zeps. The Zeps will adhere to a rough surface much better than a smooth surface. It will want to peel and flake if the surface is smooth or applied to thick. Up until my MH was 16 years old I could still hand polish it with a lot of effort. Two years ago I applied the Zeps and it still looks good. Some folks in AZ for example say it is effected by the hot sun and will yellow and flake. Machine buffing is a lot of work to bring back the shine and you have to be careful not to burn through the gel coat. The front and rear caps have a thicker gel coat maybe .020. The sides especially older Filon are very thin, you can actually see the fiber pattern through the resin. You don't want to expose the fibers buffing the surface. My conclusion at 68 years old and two repaired rotator cuffs the the Zeps was easier. If I was 48 I would try machine polishing with a good cutting compound first then a polishing compound followed by a good wax.