I also use Duratec's grey surfacing primer, it buffs out beautifully.
I'd start out and board sand with 80-100-120 and 3M's dry guide coat, so you know what is flat, what is low, and what just needs help.
Mark the lows, and shoot two passes of Duratec into them first. When that has tacked, shoot a pass of 20 wet mils... Which is just about as much as will hold on to a vertical. If you are using a conventional cup gun or gravity gun, its going to take a lot of product due to the thickness and overspray. I use a gelcoat
gun for the first shot to get a good build on the surface. You can use foam weenie rollers, but watch that you don't over work the surface, there is a point of no return that once the product begins to gel you start making non-skid texture. I suggest if you roll, do not do it in direct sunlight.
Once you have the surfacing coat on...
I use a national detroit 8 inch mud hog, with 320 grit and work the flat surfaces, same as you would an automotive paint
job. Curved surfaces I've got a Hutchins Orbital long board that works well... Then what is left gets a 16 inch hand sanding
board, 3M hard rubber sanding
blocks, and the occasional paint
stir stick in a bucket of water
so it bends nice and easy. I try my best to avoid DA sanders, as they aren't big enough not to put swirls in the surface, and they fit into low spots much better than any of the other things listed.
I use a lot of red scotch brite and foam backed velcro sanding blocks to scuff out low spots before shooting the next coat on. I tend to shoot a heavy coat first, and block down flat until I start to see highs, spot in the lows until I'm 80% straight block it out and then shoot a full cover coat. Keep your pencil handy and don't "paint outside the lines."
I use Adtech's P77, for deep lows. On top of that I like and 3M's Dolphin Glaze as it fills the pores and feather edges. P77 feather edges better than most, 3M's Dolphin Glaze is one of the best. They don't shrink and sand about the same texture as gelcoat
and Duratec which means you don't end up with high spots that are made of a harder product than the Duratec.
Shoot on your final coat, I normally go 10 mils with a gravity feed gun and focus on laying down the perfect coat with as little orange peel as possible. (Don't get the gun so close in the corners that the air flow wrinkles the product...)
When that has cured, wet sand it out to 800 grit, and power buff it with Aqua Buff 2 and you can see yourself in the surface. Normally this is a two-three pass deal. You have enough product on that you can sand through the imperfections, so the first time you buff it you'll see more stuff than you could feel.
I don't like shooting on much more than 10 mils wet for the final coat, namely you can make high spots that take forever to block out with 800 grit... but if you drop down to coarser grits, you are apt to burn back through. More elbow
grease, flatter finished surface, and no "haze" from sanding scratches because you never put any in the surface...
The biggest thing to keep in mind is that there is a difference between a smooth surface and a fair surface. You can get something shiny with a bunch of waves and ripples in the surface, which means that you either apply a lot of horsepower and a lot of time with a fine grit and don't pick on any one spot... Or you spend a lot of time with 80-120 grit and don't pick on any one spot and spray a lot of filler covering scratches.
If you don't spot fill your lows, and continually shoot a full coverage coat on you have to sand the thickness of the product off the entire surface that is not low, in order to bring the low spots up to level. If you don't sand the entire thickness, of the depth
of the low spot off the surface... You still have a low spot!
That is why aiming for 80% perfectly smooth and flat works well as you can fill the lows while working down the surrounding high spots. It ends up looking a bit like a patch work quilt, but with a flat piece of aluminum
angle and a bright light you can mark out all the lows that look like the shape of texas
and either putty them or roll another round of Duratec on them.
After the final coat of surfacer, the final sanding... You aren't trying to sand off anything other than the surface imperfections in the coating. The orange peel and the like. Once you buff it, you end up glass smooth and fair.