The Schrade USA (Schrade Imperial) sharpening instructions from 1991 say to sharpen the edge from 10-15 degrees per side. It is a pleasant surprise that a practical angle is (was) stated; not the usual obtuse 22 or 20-30 degrees. The knife that the instructions accompanied is a liner lock folding hunter with a thin edge and a fine point. It has the older brass style liner lock that says "press." The sharpening instructions also state that the edge should be finished on a fine hone after an initial roughing on a coarse stone. The information is simple, yet practical, and almost never seen with modern folders. A possible reason for this is that Schrade produced its own products in-house. A copy editor could actually ask someone a practical question.
The sharpening instructions from Eye Brand (Carl Schlieper) warn: "never sharpen your blade flat as this will ruin your knife and void any warranty." Aside from avoiding superficial scratches, this is a very pessimistic statement. The same instructions say to use a sharpening steel at a high angle (20-30 degrees), this would produce a much poorer edge than flat (or very low angle) grinding.
The sharpening instructions from Eye Brand (Carl Schlieper) warn: "never sharpen your blade flat as this will ruin your knife and void any warranty." Aside from avoiding superficial scratches, this is a very pessimistic statement. The same instructions say to use a sharpening steel at a high angle (20-30 degrees), this would produce a much poorer edge than flat (or very low angle) grinding.