First off, what is the difference between Mr Surfacer 500, 1000 and 1200? Is it grit size or something else?
Secondly, what grit sandpaper do you use to obtain an almost glasslike (or even glasslike) final sanding on models?
The lower the number, the thicker and coarser the finish… for filling a rough surface you would use 500… for a super finish go with 1200 or 1500…
http://www.missing-lynx.com/reviews/other/mrsurfacerreviewbg_1.html
Hmmm: For your last question–2500 Grit WET!
Micro-Mesh sanding pads and sheets range from 1500 to 12000. You can achieve a mirror-like finish. Use wet and don’t skip a number.
Available from many online hobby retailers, I have found a set also available at Rockler woodworking supply or Woodcraft
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I use the MicroMesh pads, sticks, and sheets, depending on the situation.