I use a Princess Tire Undercoating gun $20. for my Fluid film. per the website FAQ I thin it a bit with cooking oil .
"I have a gallon of FLUID FILM and want to thin it down. How can I do that?
The first recommendation is to thoroughly mix the product. FLUID FILM is thixotropic, and will get heavier upon sitting. Achieving room temperature with the product will help as well. If a thinner version is wanted, start by mixing a small amount of vegetable oil to the product until the desired consistency is reached. "
Another product I use per internet research is chainsaw bar oil as a touchup.