Rough up the existing paint good getting the loose stuff off and the just primer the bare areas. If the paint is still sticking after all these years then the chances of you getting better adhesion after fully stripping the paint is slime to none (They ALL look good after a fresh paint job, it's what it looks like after a few years of hard use that's the real factor)
I'd avoid sprays cans for anything except primer or flat paints. Spraying it on is probably the fastest and easiest but not necessarily the best way. Often it is not the most practical way either because of overspray problems (It gets everywhere and I do mean everywhere). Rolling and tipping, if done properly, is probably the best way. You get a better bond and thicker coats without sagging than you do with spray paint. Downside is it's more work and takes more time than spraying. Plus side is you don't have to spend the next year getting paint dust off all the stuff in your garage
