36 vs. 80

I use 36 all the time on a mudhog. :) If you have multiple layers it's not an issue just get it down to the final layer and then you can finish with something finer like 80. If you're worried about primer not filling 36 it's not going to fill 80 either unless you're using a poly. Also if you know the area is going to need filler no point in using 80. I do almost all of my mud work with 36 grit... The faster you take something off the straighter it will be.