> That said, you could just put zinc oxide sunblock on your helmet and then protect the UV-blocking layer with a spray urethane top-coat, and thereby extend the useful lifespan of your helmet by years.
Please don't fuck about with safety critical equipment.
I do not accept as axiomatic that bike helmets are "safety critical". They may be safety-improving, but I have no evidence available as to how much.
While following up, I discovered that what I suggested is patented. US 6884501.
Anyway, please don't ask people not to hack on a site with "Hacker" in the title. You are free to replace your own expanded-polystyrene bike helmets on a schedule set by helmet manufacturers and cyclist-targeting publications, if you choose. I believe that if a helmet shows no outward signs of degradation or damage, and no known incidences of impacts, there is no particular need to replace it. Just use it until it's dirty, damaged, or unfashionable, and replace it then.
There aren't a lot of studies on the real-world effectiveness of factory-new bike helmets, and as far as I know, none that actually support any particular expiration date for them. Even Snell admits that their 5-year guideline is based on "consensus" and "prudence", and not any actual evidence [0]. And one of the arguments is literally that "we will probably have better helmets available five years from now". Having been around longer than that, it just isn't true (at the price point where I buy helmets). The bike helmet I have now is largely the same as the one I had as a kid. More holes. More fashionable. More comfortable. Functionally the same. Wrap skull in 2" shell of expanded polystyrene, and put a cover over it.
Keep your helmet cool, clean, and out of direct sunlight, and it will last longer than five years. Prophylactically treating one of the well-known causes of failure in expanded polystyrene, immediately after purchase, will help.
Anyone who cycles frequently will likely replace more often based on acquired grime or stink, anyway.
Please don't fuck about with safety critical equipment.