supra90turbo said:
...you need at least a 50ra surface
Someone else stated it in SF like that too, as if the measurement worked like the way sandpaper is measured/rated.
I think it is the other way around. To use a Cometic you need
less than 50 RA.
Sandpaper is measure in:--> # ofabrasive particles/spare inch area
RA (rougness average):--> "average of all peaks and valleys. Lower numbers indicate a smooth finish"
From
http://www.ljstar.com/Default.asp
or
http://www.measurit.com/FAQ/RA.html
for those interested in other measurements:
Surface roughness is calculated by a profilometer using any of four common methods: Ra, RMS, Rz, and Rt (Rmax).
A. Ra or roughness average is the average of peak and valley distances measured along the centerline of one cutoff length (usually 0.03 in.).
B. RMS or root mean square is an older method, not common today, averaging only the heights of all points measured in one cutoff length. Readings are similar to Ra but about 10% higher.
C. Rz, a 10-point average, is an average of the five highest peaks and the five lowest valleys measured in one cutoff length.
D. Rt (Rmax), maximum height between peak and valley, is the value of the vertical distance between the highest peak and lowest valley measured along one cutoff length.
