How to polish/sand Future?

[kenno3]

Hi, what can be used to sand a layer of Future, so that it is polished very smooth, without water-lines or paint-lines?

I painted on many layers of Future onto a gray model with tiny scratches, using a fine paintbrush (smaller than a cotton cue-tip).
But there are tiny water-lines from the liquid Future not drying evenly.
I need the surface to be very very flat and smooth for transparent clarity (think of the surface of a diamond), since I will use this part to make a mold.

Since Future is transparent, I cant see if the Future filled in the scratches or not. I made sanding sticks out of Popsicle sticks and glued 2500 grit sandpaper to them, and sanded the surfaces before applying Future.
But 2500 scratches the Future, and 2500 is the highest grit I have.

I have Novus 1,2 and 3 but I think 2 is too rough and not sure if 1 has any grit.

And I read about toothpaste but im not sure what grit it is.

So what should I use as a sanding polisher for Future, so its strong enough to sand out water marks, but not strong enough to sand the whole Future layer off?

I also tried using a cotton cue-tip to "buff" the Future surface, but I cant visually see if it made a difference.

Should I glue normal white paper to the Popsicle stick and try sanding with paper? (grit of paper =?)

And how can I verify the surface is perfectly smooth AKA the Future filled the cracks and tiny indentations (which sanding wasnt able to level out)?
I tried pressing a piece of clay against the surface, but the scratch is so small, its hard to tell from looking at the clay imprint, if the surface is actually flat.

PS: by "sand," "buff" and "polish," I mean the same thing = remove material but in tiny amounts for a finishing grade surface.

PPS: I cant use more gray primmer to fill in the scratches (instead of Future), because the primmer is too thick and changes the surface too much, which would require more sanding, which might produce more scratches somewhere else.
So I thought using Future would be easier to fine tune the surface flatness. The only problem is Future is see through, so I cant tell if it worked. And I had a problem with primmer sticking to the surfaces after Future was applied.

Thank you.