The thing about ski bases is that they should be flat, smooth and have a structure. In an overall ski tuning routine, this step comes after fixing gouges in the base, before taking care of the edges.
New skis or second hand skis or skis you have used for a while, will not have the bases perfectly flat.
Not having a base perfectly flat has these side effects:
If the base is convex, i.e. the middle is higher than the edges, that's easy to fix with sandpaper or a sharp scraper like the Ski Visions tool.
If the base is concave, i.e. the edges are higher than the middle, then you have more work ahead, because you have to wear down the edges. It is important that the skis be flat under the bindings and just ahead/behind these, if you're looking to save some work. However, a stiff race ski will have pressure also at the tip and tail, so get these as flat as possible as well.
To find out how the base is, use a "true bar", a bar of metal as flat as possible. Use the side of an edge guide with caution - these are not perfectly flat.
Get a strong light on one side and, looking from the other side, slide the true bar along the base, noting where the base is convex, concave or flat, based on the amount of light shining under the bar. Use a marker on the edges to mark those sections.
This is your first choice, since base work is slow, hard and quite risky. Make sure the shop will use a stone grinder, not a sander. Also, see who's in the back... If you see some careless teenagers, you might as well file them yourself.
If you don't feel like grinding the skis, then read on...
If the skis are pretty edge high, you can file them first in those sections. Use a mill bastard file, tie it together with another file so they are certainly flat and go lightly from tip to tail in the sections where the edge is really high.
Be careful and clean. Clean the files and the bases often, or bits of stuff will leave deep scratches. Do file in one direction only, from tip to tail.
When the base is close to flat, you'll know, because you were paying attention, right? Some may not feel comfortable filing the base and just use sandpaper all the way - same result with more, slower but safer work.
Start with 150 grit strips. I have seen both silicon carbide and aluminum oxide recommended. Stay away from the cheap garnet sandpaper - the grit comes loose and encrusted in the base.
You can wrap it on a base guide or a true bar. I get great results with it wrapped around a round 1 inch thick dowel - as you can constantly rotate the dowel and expose new sandpaper. Try more methods and see which you like.
Paying attention is critical, because you don't want to dull the edges or take too much base material. When you g it flat, switch to 220 grit and do a few passes. When you see the sandpaper cutting evenly across the base, start re-checking often with the true bar.
Keep it clean: brush with a brass brush and some fibertex pad frequently to clean the filings. Also,be prepared to use lots of sandpaper, as it will clog quickly with the plastic off the base.
Also, make sure you are sanding in one direction only: from tip to tail.
Between files and sandpaper, use a brass brush and take away any metal particles that are on the base.
At the end, after the sandpaper, start with a steel brush, followed by a brass cleaning brush, 10 passes each. Alternate the metal brush with fibertex pads, to remove the "hairs" that the abrasive sandpaper pulled out of the base (or, more accurately, ripped out of the base)
At this point you should re-do the base edge bevels - if the skis were properly tuned before, just touch up with 200 and 400 diamond stones, otherwise start with a file.
Hot scraping is next. You'll have to do it a few times, until the scrapings are clean - they will pull all the mess from the ski base and, after so much work, there will be quite some stuff in there.
Some recommend hot waxing with a hard wax and scraping it after a few minutes - might get rid of some hairs faster that way.
Keep in mind that as you're opening up the structure of the base, it will start to oxidize, which will negatively impact wax absorption, apparently. So, plan to finish the base work in one evening, including edges, hot scraping and saturating the base, at the end. Start early, work one ski to finish.
I start with this kind of work way before the season starts, unpacking each pair of skis and doing their bases and everything, about one ski per evening.
If you do have to stop for a while, just rub on lightly a block of soft wax and scrape it lightly: this should cover the base and not let it oxidize overnight.
If the base is so concave that you can't make it flat without griding it all, divide it into three sections (across) and make sure the outer thirds are flat, even if the middle is still a bit concave - this will help with the handling.Ski Racing mag 2009
Read on for even more Ski Tuning Ideas.