Flattening an Oil Stone

I have a Norton combination coarse/medium crystolon oil stone that I use as part of my tool sharpening routine.  I have spent a lot of time rough shaping blades on old tools I've restored on the coarse side of this stone, and it has become dished out from that heavy use. 

 

Now the truth is, I don't care if I have a bit of a dish in my oil stones. This just puts a slight camber or curve in the iron at the cutting edge, and in my opinion this is not a bad thing and in fact can be good.  Smoothing plane irons often have the corners ground off or a slight camber intentionally shaped into the blade to keep the corners from digging in during use.  A small amount of dish in your sharpening stones gives you this for free.  The problem comes in when your different grit stones get differing levels of use.  In my case the coarse stone probably had 2 or 3 times the wear of the medium side and my final stone which is a soft Arkansas.  This means that you shape the blade to the coarse stone in step one, putting the camber in, then have to work again in the following stages to take some of the camber back out to match the finer stones.  So I needed to flatten my coarse stone so it would better match the other two.

I had seen a post on a youtube channel called wranglerstar where he flattens some small oilstones using silicon carbide powder and a piece of glass.  Upon further searching, information on how to flatten an oilstone is surprisingly scant on the internet, so I decided to try this method.  Just a quick disclaimer - I do not endorse all the religious and or social opinions expressed on the wranglerstar channel, but I do find value in lots of the how-to content.  Here is the video that got me started with this method: https://www.youtube.com/watch?feature=player_detailpage&v=BHtDPikL810#t=124
 (the relevant part starts at about 2:05).

I started by marking a crosshatch pattern in pencil on my sharpening stone to help see my progress.

The silicon carbide powder I got on Amazon.  Just search for 80 grit silicon carbide powder.  The piece of glass I had from the days before I had oil stones when I used to use sandpaper and glass to sharpen tools.

Start by mixing a teaspoon or so of the powder with some water to form a slurry.

Then just rub the stone around on the piece of glass.

Check your progress often, looking for signs of the crosshatch marks. 

The stone will sometimes get so covered in slurry that it's hard to tell if you've finished.  In this case, just wash the stone in hot water and mark it again.  You're finished when there aren't any marks left and when the stone appears uniform again.  In my case this went really quick - between 5 and 10 minutes total time on the glass.

In no time the stone is back to doing what it does best - sharpening!

When I first saw this method I thought to myself that this would surely just grind a dish into the glass quickly which might prevent it from ever getting the stone flat.  Here you can see the glass after I finished.  It did scratch up the surface, but it made no noticeable change in the flatness of the glass.  

I'd estimate you could maybe flatten this stone at least 30 or 50 times before you did enough damage to the glass to matter.  By that time your stone may be worn down to nothing anyway.  Happy flattening!