So I just bought a jar of this Mr. Base White 1000 and I'm wondering how to use it. It is basically just a primer? Do I spray it? If so what pressure is typically used and how do I thin it if it needs to be thinned?

I haven't found much difference between base white and mr. surfacer aside from the color and I haven't seen a base white other than 1000 grit. Typically I use the Mr. Hobby Levelling Thinner or the Mr. Hobby Color Thinner. The mixture would be around 1:1 for the paint and thinner. Pressure would be somewhere between 20-25psi on a 0.35-0.5 nozzle

Uses for mr surfacer can be for priming, texturing, or filling but i think the 1000 grit of the base white is too fine. You'll be better off using mr surfacer 500 or a dissolved putty

Base white is useful if you need to start off with a light base color especially if the plastic is a dark color like black or dark green. I personally use it for aircraft and cars.