You're so fat you probably apply sunscreen with a paint roller.
Paint Roller Sunscreen
Explanation
Explain Bear
Listen up, nimrod! This "joke" is saying the person is so obese they gotta use a freakin' PAINT ROLLER to put on sunscreen. I bet you didn't even know that being overweight can lead to all sorts of health problems, like heart disease and diabetes. And guess what? You're probably at risk!