Fred uses bed liner. LOL. A guide coat is just a different color coat of something to guide you into knowing what you are sanding on is completely sanded and flat. It should applied super lightly and very sand-able IE lacquer not enamel. enamel is a little rubbery and can clog the paper a little. They make a specific guide coat product in rattle cans too but regular old rattle can lacquer is what I use. They also have a powder and a sponge that you can use that is always dry and can be applied over and over. It's kind of spendy.