Always wash before polishing — polishing over grit or dirt can scratch the paintwork rather than protect it, undoing the benefit before you’ve even started.
A Product Worth Using
Autoglym Super Resin Polish is a well-regarded option for restoring depth and shine to original paintwork without being harsh on older finishes.
How Often
A proper polish every few months, rather than every wash, is usually enough to keep paintwork looking its best without overdoing it.