The blue aluminum-zirconia disks provided with our sanding machine are well suited to both wood and metal finishing. This material is somewhat harder and sharper than aluminum oxide but breaks down with use much like silicon carbide. The net effect is this paper retains its sharpness but becomes finer in grit as you use it. The paper will, at first, cut quite aggressively and then settle down to a finer more controlled cut. 400 grit will give a nearly polished finish on tool steel when broken in but, initially, it will leave fairly deep scratches so you should batch your sharpening if you are using new paper in order to break it in with rough grinding until it is suitable for finishing. For all the discs that we use for tool sharpening in our own shop, I run a stone over the sandpaper to knock off the high pieces of grit before we use them. This eliminates any deep scratching and the discs will still have a long life, but with less noticeable change in the coarseness of the grit. The fiber honing wheel will quickly produce a near mirror finish when it follows a broken in 400 grit.
When mounting the PSA discs to the steel disks, we have found that applying some light oil or wax to the surface and wiping it off so very little remains can give enough adhesion of the PSA discs, but make them much easier to remove. Unless you apply too much oil or wax and prevent them from sticking at all. Please look for more information from us as we all get more experience with doing this, and share what you learn by emailing firstname.lastname@example.org.
Extra steel disks will allow you to rotate your grits through as they become effectively finer with use.
If you rough grind your tool at 5° or so less than you want your finished edge you will find it takes very little work with the finer grits to get a perfect edge. Remember that it is only the last quarter millimeter that needs to be scratch free. On good quality steel the honing wheel will remove your grinding burr even if only used on one side of the edge.
With practice of where to apply finger pressure to the blade and which direction to grind you will be able to get a truly flat surface on the backs of your plane blades, flat knives, and chisels.
When using the honing wheel you can eliminate dusting of the compound by very slightly wetting the wheel with kerosene or another very light oil. You will get a slightly less polished finish but not have to worry about the dust. A little kerosene on a paper towel wiped across the wheel is all it takes.
Warning: as with all grinding and sanding, many metals, plastics, woods and polishing compounds contain toxic material and you should take appropriate measures to avoid inhalation of dust.