Perimeter vs. Area

Sandra has just bought her first house and it is a bit of a fixer upper. Sandra believes that a new coat of paint is exactly what her new home needs. However, Sandra has never even had to paint a wall before so she has no idea how to measure out the amount of paint she needs to complete this project. Should Sandra find the surface area of the walls or the perimeter of her house to measure the amount of paint she will need?