I think you have the right idea. The headliner is probably clean enough to spray. It's not like people will be spilling drinks on it or dropping french fries on it. I will only say two things; Mask, Mask, Mask and wear a good mask. Don't just wear a dust filtering mask, wear one that will filter out the bad fumes. You will be trapped inside the car with the fumes. I will guarantee that you will spend way more time prepping than doing the actual spraying. Send us some before and after pictures.