Yes, you should. You should always pay a painter before they start working on your house. This will ensure that they are motivated to do a good job and that you won't have to worry about them doing a poor job or disappearing with your money. Plus, it's just good business practice.
No, you should not pay any amount to a painter before giving them a contract. In fact, you should always hire a professional painter through a reputable company. By doing so, you can be sure that the job will be done correctly and to your satisfaction. Additionally, most painting companies offer guarantees on their work, so you can be assured that any problems that may arise will be rectified at no additional cost to you.