I’m going to paint the inside of my unit. It’s all brick. I no what to use I just can’t find out the quantity needed. I’m using 3 step for undercoating and wash and wear for top coat. I’ve been given three different quantises so far from 12lts of undercoat to 4 lts . The length of the room is 8 mts x 3 mts by 2.44 high. I m not worried about the door or windows if you could give me an idea thanks.