As a 'glider sander' and surface contourer I usually stop at 400 or
600
grit
and find the performance at least equal to a highly polished surface.
There
is some wind tunnel data that suggests a slightly rough surface actually

keeps the boundary layer energized for greater laminar flow whereas a
glossy
surface trips the flow to turbulent sooner.
Bill D

Hi Bill,
I have not seen any research data as it relates to sanding.
When sanding, it appears, mixing of air takes place sooner and laminar
flow is reduced.
Sanding with 360 to 400 grit works in some in some cases with the FX 67
and similar airfoils. It is just an other form of a turbulator in my
opinion. In the case of sanding, the run of laminar flow is reduced,
because of that a more energetic transition can takes place and attached
turbulent flow can be obtained in an otherwise unfavourable pressure
recovery as with the FX 67. The same can be achieved by placing
turbulator bumps at about 40% and 50% chord with a 1" inch spacing, the
bump is 3/16"diameter and .025" high, this allows one to keep the
polished surface, plus a positive flow trip. If there is a vestige of
laminar flow left in the region, it can move in-between the bumps. At
climb speed the wake spreads at 35 degree, give or take. When cruising,
the wake pattern is more parallel at first and then spreads.
I have used this technique to solve some short comings on my original
HP18. In either case it is not optimum I rather see
the airfoil fixed or at least corrected so it meets it intended
design criteria.
Similarly, HP18 owners complained about wing drops on there take off
rolls. I recommended to place a turbulator 50% chord on the top surface,
within the span of the aileron. which stop the wing drop problem.

Udo