100 grit rule

unless he means to finish your putty work with at least 100 grit before primer,maybe?I finish my putty work with 180,and finish my primer with at least 400 wet,if im spraying a solid(non metallic)color,or 600,for a metallic color,to keep the sand scratches from showing through.Ive been doing paint and bopy work for over 20 years,never heard of the 100 grit rule,but then again,im pretty much self taught,lol