I prefer a natural horse hair paint brush and a small painters cup. You can find them at most home improvement stores, or we have them available through our shop if you need.
The reason I like the brush as an applicator is that there is less waste of prep, you don't have cross contamination when re-using with other preps, less need for wearing gloves, less chance of spilling, no more dunking a wax pad into a Tupperware container of long forgotten what prep, less evaporation, more precise application, etc etc.
When you're done prepping, simply pour your prep back into it's original container.