Replying to Protecting Signatures on Helmet?

Hey NS,
I recently got a bunch of signatures from a bunch of pros on my helmet. It was a very expensive helmet so before you say, "Just put it in a box in your closet." I really can't because I just bought it last year and I don't have the money to buy a brand new one. Plus I still want to use the helmet anyway. I was just wondering if there was a way to protect the signatures, something that won't fade, smear or take off the signature (obviously) and won't wreck the graphics on the helmet itself. The signature was done in sharpie, not sure if it matters or not. I've heard people say clear coat nail polish, clear coat spray paint, mod podge, etc is a good way but idk.