If you use airplane dope, be sure it is the original type Butyrate dope. Some newer Dopes have clear urethane instead and will gum up when exposed to gasoline. And yes the original Butyrate dope is also ethanol proof. Sand the cork lightly, but you don’t need to remove all the old varnish sealant. Don’t need to dip the float. Just brush on several coats and use acetone to clean the brush or to thin the dope if needed. Don’t worry about using too much dope. It is very light when it dries. That’s why it is used on airplanes. Crazy glue/super glue also works but it is much heavier than Dope so must be used sparingly if at all.