I doubt you will have a problem with rust since you have that much por 15 underneath as I have also ,and I topped mine with por 15 glisten pc. Por 15 recommend's this for proper adhesion for the spatter paint. Rust don't start on outside in,it start's inside out with pin hole's on bare metal . The trunk spatter paint is water soluble so it will let moisture go through and if no proper sealant underneath like rust preventer it will start to rust. Yes it is a good idea to clear the spatter to protect from dissolving it from moisture and they recommend that. The zolo tone sound's like a good product much mote expensive with the proper prep product's but they also say it has to have a proper base primer sealant coat before applying in two stage process. It will only adhere to what it is sprayed on ,same principle as a good car finish ,proper prep.