I normally don't get burned easily and never wear sunscreen back home or even in Chiang Mai (granted I'm mostly in doors in Chiang Mai)
But my shoulders and upper back actually got burned from walking in George Town for about 3 hours in the afternoon when I went there last month (not severely, but enough to hurt and have little blisters and peeled skins a few days later)
I ended up getting an SPF 130 sunscreen for the rest of my trip there