You will need to file a claim against the security deposit within 30 days of the tenant leaving. If the walls were freshly painted and the damage requires a complete repainting, that is a valid charge against the security deposit for the full amount. You should take pictures before and after to show the work was required and performed. I'm not sure how you're getting to $900 if you are not using a professional painter. You cannot charge the tenant for your time unless you are a licensed painter.
This communication is not intended to create an attorney/client relationship. It is always recommended you consult an attorney in person to discuss your case. The Law Offices of Stage & Associates practices state-wide and represents homeowners and community associations. Please visit our website at www.stagelaw.com.