i have a 25 foot boat trailer with surface rust on each end and paint in the middle. i need to know what 2 products would be best to use and how much? pint or quart? i was thinking Rust Encapsulator and Chassis Black. will this work on the painted surface or will it have to sanded? it is original 25 year old paint. rust is on about 10 feet in front and 5 feet in back and comes off easy with wire brush. thanks for any help and advice.