Down - Could you expand on the long boarding a little. What kind of imperfections is this stage aimed at? Is is the small stuff like what would look like scratches or a rough texture, or is it to beat down long waviness, like what would be produced from a hard spot at a stringer or bulkhead?
The long boards are for fairing. DA sanders are for smoothing. Fairing would be taking any dips or gouges out. If your rotator cuffs are in good shape it will speed up sanding when you prime the entire topsides. Technique again and for me an adjustable height platform to work from. The truth is a 30' boat's topsides amount to about the surface area of 4 sheets of plywood placed end to end. Not such a big area. An 80' Swan is a little bigger
I do it this way:
A working platform that can be adjusted to keep my shoulders at the level of the section I am boarding. LITTLE REACHING UP OR DOWN. Chest high so you can "lean into it comfortably". Back and forth with the two handed board around 30 degrees to horizontal. The distance between the handles matters too. I make my own simple boards. Their length and the handle locations are adjusted to my comfort. They are usually about 3' long. That can vary depending on abrasive availability and the surface condition. You can make a 2 man board, too. The other "guy" should be about your size!
A day of this is plenty! My arms are folded in front of my chest for part of the stroke. My hands are shoulder width apart. Extending as far as comfortable on each stroke. Rocking back and forth front foot to back foot establishes a rhythm and adds to the momentum. Switch direction, hands and feet. 30 degree scratches intersecting the previous ones in the opposite direction. Intersecting scratch patterns cut material much faster. KEEP THE BOARD FLAT AGAINST THE HULL FOR ITS ENTIRE LENGTH. Adjust the platform height and make another pass. Repeat as necessary. Remember it is a labor of love!
You can gain, boarding down to 220. Stepping down your abrasive grits is a judgment call. Depending on how much fairing is required starting with fairly coarse grits will shorten the time. It is a lot of work trying to remove the scratches from 50 grit with anything finer than 80 grit. Don't move to, too fine a next grit in the hope it will save time.
When the eye tells you things are right and your hand feels things are right, you are ready for the dual action sander. Same working platform. A figure 8 overlapping pattern as you work over the surface from one end to the other. This should involve no more pressure than what is required to hold the sander against the surface. Learn to look at the surface.
Depending on how bad things were or how good things are, alternation the primer colors slightly will give you a visual gauge of the surface condition as you sand. This can be a very subtle change in color that you create by having a small amount of the same primer with a different color to tint the base of your primary, primer. A job that requires fairing gets a final prime coat after fairing is done. If you get it on nice and smooth (you can roll and tip the primer, too) a DA sander will do the final polishing. If the final primer color is the same as the topcoat it will increase your chances of success. I always try to use a primer that is close to my topcoat color. For a red top coat that is particularly important. Each color covers differently. White is the easiest.
I am at the point where the calories I burn doing this become part of the record! Don't start if you have a heart condition!
Wear dust masks that work and eye protection. I suffer with a full face respirator even for the sanding.