I always seem to burn worse if wearing a full face helmet. With no helmet or a shorty (police style no face shield) I can usually go about 2-3 hours without burning. With full face about 1/2 hour, the shied acts like a magnifying glass. (these times are doubled when I go somewhere at lower (under 4K feet) altitude.)
I use the "waterproof" kids sunscreen (spf of at the very least 45, this year I actually found some spf 60) and Kroger brand chapstick (spf 30, the highest I have found) from City Market.
Two burning problems I have are
1) skin sunburning on face between beard hairs (can't get sunscreen there)
2) stopping and walking around, with helmet off, no hat, and forgetting sunscreen on bald spot (helmets really hurt to wear with a sunburnt head)