If you have unsocialized rats, one of the best ways to help them gain trust is to bring them into your shirt, against your skin. This allows them to feel save inside the "cave", and as they inevitably touch you, they experience you do not hurt. They learn to like it inside the shirt as a hidey place, and they learn you are part of that safety. Use any shirt you don't mind getting some wee or poo on, button the shirtsleeves or tie them to your arms, plan to wash yourself off later, and let them in. Do this both in groups and singly. As your rats lose their fear, they will gain better control over the wee and poo, and because they naturally prefer not to make mistakes, they will get better at holding themselves until you return them to the cage. If you can leave them in your shirt for half an hour at a time, all the better. Exposure exposure exposure.