I applied SPF 100 sport spray, every hour (give or take) for roughly 4 hours. I was hanging out at the lake and at slightly higher elevations so the water reflection and elevation probably increased the damage.
I pretty much had no idea that I had a problem until I scratched an itch on my back and it hurt really bad. I considered for a split second that I had a burn but like an idiot, I rationalized it as "sunblock burned the broken skin from the scratch".
The blisters formed by the next day and I started reading the can to see if I missed something about applying it. I found the customer service number and called it. Before talking to anyone, I heard a recording tell me that the FDA doesn't require expiration dates on sunblock and to consider it expired after 3 years. This bottle was easily 4-5 years old.
I had no clue.
I went to the doc and got some amazing medicated gauze and they sent me home with pain meds after cleaning the burn off and showing everyone in the building.
It was easily one of the most painful things I have ever experienced. Except when my tank top straps got dried to my blisters on my shoulders and became part of the blister scabs overnight. Trying to pull them off was almost blackout pain. I soaked in a tub for a while until they basically melted off.
This was preventable- Sunblock expires, please check yours and make sure you store it according to the instructions.
EDIT- I want to clear up: I'm naturally, very pale person, I was at high altitude while on and in the water. I do not blame anyone but myself for this. Whether expired or I stored it incorrectly whatever it was- it was my own fault. I should have been careful with making sure the things I use to protect my skin were being used as they should have been. I agree that all sunblock should have expiration dates, I just don't blame anyone but myself for my lack of due diligence. This isn't a chemical burn, just a bad burn on skin that rarely sees the sun with a few contributing circumstances that made it worse.
I'm not planning to sue anyone although I appreciate and understand why it is being suggested. I just want people to have all the information to protect their skin! Be safe! Skin cancer is a real danger.
TL;DR- Sunblock expires.