The very first time you do it, it’s going to sting, because you’ve exposed your skin to new chemicals. With continued use, your skin will become accustomed to it, and so long as you apply it correctly, you won’t break out in a rash or get burned much at all.
I have very course hair, so I need to leave it applied for at least 5 minutes, preferably the full 10, in order to get everything. The only areas I still get very mild rashes/burns are the leg-hip creases, and small little pinpoint areas here and there along the edge of the lip folds (on the underside, which you should NEVER use Nair on!!!) due to very slight bits of the cream slipping down a tad bit.
As for the back-end; again, I have super coarse hair, however it doesn’t need as much time to remove as the front. If it starts to sting, you’re good to remove it. Don’t worry about your anus; so long as you haven’t messed with it recently, nothing will get inside there and it’ll be just fine. You should be more concerned about the upper-end of your crack; that’s where the burns are more likely to get you. Anywhere that your skin presses against another area of skin is at higher risk for rash/burn. As for everywhere else on the butt, the hair back there doesn’t grow in as thick as it does other areas, so you shouldn’t need much more than a thin layer of cream; put enough on that you can see a color change, but don’t put so much on that you can’t see the hairs anymore.
My front, back, and crack are all just fine, save the very mild burns in the aforementioned areas. Just leave the areas alone for a day or two and it’ll heal on its own. If it stings, I suggest an ice cube. Works great. Aloe lotion helps too.