I ended up with a lot of flat sawn strips in my boat, and they are a bugger to sand. The ring lines are harder than the sapwood, and a power sander hollows out the sapwood in a hurry. Using 40 grit on the orbital to kept things in check, and a lot of hand sanding was needed to keep the boards smooth. I will not use flat sawn boards again for what it is worth.

Pete

Some woods have very hard grain lines; I agree. Were you sanding cedar? Also, a lot of sanders seem to come with too-soft (IMO) pads. Combine that with no-longer-sharp paper and you can certainly hollow out the softer grain.
I don't understand your comment about sawing.
How did you rip your strips from the boards?
Usually a flat-sawn board won't produce flat-sawn strips for me if I'm just ripping strips from the edge of the board.
And, vice-versa - a quarter-sawn vertical-grain board ripped into strips will produce flat-sawn strips (which would be a shame unless you were aiming for a specific visual effect from flat-sawn wood, as those vertical grain -aka spar quality or instrument quality- boards are usually expensive).
Like Bill Hamm, I buy cheaper flat-sawn boards and rip strips from the edges. Some folks buy 'two-by' or '4 by' (2x6, 4 x4 etc.) stock - with those you have lots of ripping options - with the right saw.