I bought the gun with wands from wool wax a while ago and use it with fluid film.
Heat up the fluid film or leave it out on a hot driveway before application.
Woolwax(R) "Pro" Undercoating Gun
I use a small 6 gallon pancake compressor set at 75-80 psi and it works well, I only need to wait a short time if the compressor needs to catch up.
Fluid film has given me the best results over the years on different trucks. I usually hit the internal parts of the frame I can snake the wand into sometime in the summer and then the exterior of the frame before winter.
Like others have said, wear old clothes.