Thing I found out about sanding is it is a "nuanced dance" with paper type, grit, the medium being sanded and a variable you only find out in the process. (but you already knew all of that).
The sanding paper will either block up, (and might as well be sanding with $20 bills), or cut too much, leaving scratches, and either re-spraying, or chasing with the next lighter paper.
You know you have to find the best paper, wet/dry, pressure and how far you can go before tossing that rag on the ground, matched to the surface/medium you have laid down, is the "formula", and it is almost always a whole new gig, with a new type of materials and tools/conditions your working with.
You and I, know, it's how it is. No surprises, but perseverance and being an anal type personality, your gonna prevail in the end.
And still, some snot nosed punk will never realize the craftsmanship and dedication to craft, that went into the thing. But you know, and some old shit will walk up, look at your ride, glance at you, and you will know, he knows.'
And, that is enough.
Keep on, keepin' on...
