Some people say it hurts most on the chest others say it's worse on the back. It is never so bad that you can't stand it, you will never exactly know, unless you make that experience on your own. The amount of healing time required will vary from person to person depending on their health as well as the type and location of the tattoo, usually 7 - 10 days. Every tattoo, old or not, can be covered with something else. The only problem is to cover a tattoo that is already really dark, because the artist can't use a darker color to go over it.
Sterilization is the most important thing in a tattoo studio.
There are a lot of tattoo studios that might look a bid filthy from the outside, but don't worry. If they are an official studio, they have to be as clean and careful about sterilization as a doctor, a dentist or a hospital. They have to use the same kind of equipment they use, for sterilizing. There are blood borne diseases such as HIV, hepatitis and syphilis that are life threatening and can be spread without adequate precautions. Professional tattoo studios have been in the forefront in implementing proper sterilization techniques for years, long before it was fashionable, as it has become recently.