Q. What’s the optimum number of knife marks per inch when planing?
A. To answer your question, I need to know, “How much sanding do you want to do and how smooth does the surface need to be?” The best for many cases is between 12 to 20 marks per inch with the higher number meaning a smooth planed surface with less sanding to be really smooth. The lower number meaning a bit rougher and perhaps an experienced eye will see knife marks if sanding is not extensive.
There is variation in species when assessing the best number of knife marks. For example, see Figure 7 in Machining and Related Characteristics of Unites States Hardwoods written by E.M.Davis in 1962 (U.S. Dept. of Air Technical Bulletin No. 1267).
When sanding, remember that moisture content and sharp sandpaper are important factors.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.