I was wondering, is 400 a fine enough grit? When it comes to diamond stones, many seem to start talking about 800 as the starting point. The thread about knives and sharpening has a few posts on the dimond stones and I don’t think any are starting at 400, that being said skis and knives are quite different items.
400 Fine enough to start the progression of diamond stones after using a fine/finishing file after coarser files before? Yes, but you’d still want to go to a finer stone to finish after the 400grit. I usually go up to at least 800grit for finishing.
I have 100 to 1500 grit diamonds and use the 400 and 800 the most. Will use a 1000 or 1500 occasionally now but they are very fine so usually more of a “race ski” type of extra step. The 800 to 1000grit step is much smaller than the difference between 200 to 400 step.
100 and 200 are pretty coarse so usually use those when I don’t have my fine file when travelling or to make quick work of rounding tips/tails above the widest taper points.
(With apologies, I ain't that great at it; I just have been at it off and on for some time, through racing also.)
To me, the reason there are such differences in where to start (anywhere from 100 to 800 and higher) is that one goal is to take off the minimal amount of edge (especially the base side) for minimizing need for base flattening grinds and ski wear: if a finer grit will do the trick to eliminate the level of edge damage/burr that is most common for your ski area, skiing, and particular damage, etc., then that's the one to use. So experiment to find what severity of burr or edge damage wants what, with grit fineness.
In addition to diamond stones, many use thin, fine stones of ceramic or actual Arkansas stone, at times. Many race shops have sold the thin ceramic stones, a white one (1200) and a brown one (600). I've found these take off less material than the comparable diamond version, and don't require water lubrication, the way the diamond stones do for longevity. Also, unlike the diamond stones, the ceramic ones don't wear out.
In particular, surprisingly, the 1200 white one works well as a starting stone for most routine burrs. (I think it has something to do with the difference between the materials, ceramic versus diamond grit.) This saves time, elbow grease, and most of all, metal base material (and metal edge material).
Last, there's a good reason that the diamond stones include that 1500. It's an answer to the question, "what is the least fine grit that will pretty much automatically guarantee no slight, intermittent hanging burr edge from using stones for tuning?" (That is, if the alternating stroke method is used, especially towards the end.)
Working by hand, as a near final step, I used to have to inspect by feel the edge for hanging burrs of whatever length, just to make sure none were there (after tuning by alternating edge side to base side single strokes, for the final, finer grit tune steps at least). Using the 1500 grit as a final step (seemingly too fine to make any difference for anyone but racers), I stopped having to check for an occasional bit of hanging burr edge here and there. It just stopped happening.