You are making a bigger deal out of this than it is. Just sand the damn thing :)
While I do try to direct sanding to the high edge, if the sander slops over a bit onto the lower, just move it away a little. If you are skeptical, just make a throwaway joint and sand it to prove to yourself that all will be fine.
Regardless of how carefully you sand, there is going to be an area that is not in the same plane as the rest; however, it is minute and not noticeable. The only way to avoid it is to take down the WHOLE assembly until the joints are flush and then finish sand. That is one of the handy features of a drum sander. Of course, when you do that, the whole assembly is a skosh thinner than you planned. No big deal, it doesn't matter...in my life, at least, a face frame etc. that winds up 47/64 hick - or even less - rather than 3/4 is perfectly fine.
That's making this a bigger issue than it is in practice.
The rectangular pad of most "finish" sanders, and the small round pad of most "compact" finish sanders, allows you to sand up to a line, or joint, with enough accuracy for 99.9% of any desired sanding results, and certainly without need of a "stop".
Mechanical sanders are tools, and like all tools, some use the same tools in different ways to get the same result.
And, like all tools, you generally get what you pay for in performance.
I use a ROS (Festool Rotex 125) basically for rough sanding (60,80,100g) and minor material removal;
The proper degree of "anality" is what I've mostly been trying to get a grasp on with these questions. As always, the "answer" is a stew of all of the various responses I get here coupled with a bit of my own sense of things.
As for sanding being fun ...
I have to say, I got a small amount of pleasure from hand-sanding the curved surfaces. But repetitive machine sanding is still a chore.
Let results be your gauge. Don't be afraid of a project not being perfect as you will certainly be over doing on many steps. Build a small and a bit complicated shop box and stain it and don't over think it. Learn from those results.
LOL, well hopefully one day you will be able to obtain equipment that works better and has stellar dust retention. When you can sand for hours, stop and walk inside and sit down with out bringing any dust it becomes more fun. ;)
Well, I certainly don't tout the Bosch for stock removal :-). I use it as a finish sander. It is less aggressive and is variable speed, so perhaps that accounts for the lack of scratches I see.
All sanders leave scratches, but by the time I get to 220 I can't see them. And I usually finish with clear shellac (SealCoat). I do work through all the grits - it seems to be faster than skipping grits.
P.S. I hand sand the shellac - a power sander melts it :-).
Absolutely, when I feel I will get better results, mainly due to the wood and grain, by doing so.
On face frames, door and drawer frames, and other butt joins, I will often start with a low angle block plane to get close enough to not affect the adjoining piece, and maybe a card scraper, then follow up with the appropriate sanding to get it to the final finished state.
And your methods very well may work better for you than they would for me. I am not sure I even own any grit past 180. ;~) And I mostly only use rub on gel varnishes and stains on occasion.
Inside corners, are inside. Don't need sanded, do they?
Really, I guess I did not get rid of everything that is not a ROS. I still have a couple triangle shaped detail sanders for inside corners and such. It still seems to save a lot of time using the ROS for all that I am able to reach.
Dust? That's what I like about my ROS. It makes lots of it. Tons. I got a motor off of a 5 HP air compressor and converted it into a ROS. It is heavy, but it will sand like a Manchurian Devil digging in the desert. Using a 220 3 phase drop cord is cumbersome, but you get used to it, in the search for the ultimate sanding job. I use 5/4" rough sawn lumber for face frames and sand it all down to 3/4". I start out with # 2 grit sandpaper, moving 1/2 grit closer to 1000 grit on every pass. Yep, that's right. Almost 2,000 different grits before I use any finish.
Nothing is more satisfying than coming out of the woodshop, and shaking like a dog, and seeing the whole neighborhood disappear under a fine coating of dust. Honey bees flee in confusion, as they can't tell what is pollen and what is dust. I also try to remember to blow my nose every time I stop sanding. You have instant wood putty for cracks and nail holes that match the wood you are working on sanding, and because it is water based, it takes stain perfectly once it gets dry. Then I immediately start up the stairs and track footprints through the house on the way to the den and my beloved LazyBoy to take a break. When I get up, I beat on the chair with a broom to get all of the dust back up into the air. After all, the multiple HEPA air filters I have are expensive and should have to do some heavy lifting in order to be justified in running 24/7. I had to upgrade to a 400 Amp service to keep from blowing the main breaker with all of these magnificent machines running all day and night.
To get the dust off of the frames, and out of the shop, I usually get the water hose out, and spray down the entire shop. I can then tell which machines are low on wax, when they immediately start to rust. Part of my preventative maintenance program. The water goes on the wood I had been sanding, of course, and raises all of the grain, which is why I sand it all to 1000 grit to start with. After the grain raises, it looks like everyone else's sanding jobs, less all of the swirls and marks that you get from all of the finish sanders and such most people use.
So there, the real secret is out. How to sand stuff before you put smooth stuff over it.
Gosh sakes, people. Experiment a little. Each wood, each sander, each project is different. Sand it until it looks good, then put some shiny stuff on it.
I hope my guide has been useful, or at least entertaining.
PS. Not too many forests were harmed in the making of these cabinets. Just
2 or 3. The particulates that went into the air are my way of combating global warming, which makes up for it.
LOL ... reminds of the time I was required to remove, as the contractor, a tree that we supposedly murdered as a result of new residential construction a few months before ... bogus, but you can't fight city hall and expect to get permits approved in a timely manner ... and yes, they actually do hold what is in effect a "tree murder" court.
A 30" in diameter pecan tree, and when the removal crew felled it, it was basically a thin shell, from roots to top branches, full of dust from years of an insect infestation, and the cloud of dust that ensued in cutting it up in pieces small enough to haul off blanketed the neighborhood for blocks.
Saw the cloud of dust as I was driving to the site, knew instinctively what had happened, and without stopping, went straight to the local car wash and purchased $200 worth of gift cards to present to the adjacent neighbors on two streets to get their vehicles washed ... turned out to have been a prudent, cost of doing business, move. ;)
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