How to flatten and sharpen
If the non-bevelled side of a plane's iron is not perfectly flat, and the cutting edge razor sharp, the plane's performance will be compromised.
An uneven blade will replicate the unevenness in the cuts it makes. A less than sharp blade will make planing more difficult, and could cause tear-out, leaving serious blemishes in the workpiece.
For the following procedures, you will need sharpening water stones in three grades - 800, 1000 and 1200 grit. They are readily available from DIY stores.
Flattening the iron
Even a newly-purchased plane iron doesn't necessarily have a perfectly flat back. Fortunately, "lapping" the back of the iron is a process you'll most likely only have to do once.
Step 1 - Prepare for lapping
Remove the iron from the plane and, if there's a chip breaker attached to the iron, remove it by undoing the stubby screw that goes through the slot of the iron into the back of the chip breaker.
You need to flatten only the first inch or so of the non-bevelled side of the iron, measured from the cutting edge of the blade.
Step 2 - Start lapping
With the area of the blade to be flattened held firmly and perfectly flat on the 800 grit stone, slide it backwards and forwards along the stone. This is called "lapping" the iron.
This action has the effect of "polishing" the iron where it is in contact with the stone.
Step 3 - Check progress
After lapping for 30 seconds or so, turn the iron over and inspect the newly "polished" area.
When the polish is consistent all the way across the iron, covering an area about 25mm (1") from the cutting edge, the iron is flat. Repeat lapping on the 800 grit stone if necessary.
Step 4 - Repeat on higher grade stones
Now repeat the lapping process on the 1000 and then the 1200 grit stone to achieve a very smooth polish.
The blade should now be flat and ready for sharpening.
Sharpening the iron
The cutting end of the iron will already have a bevel at an angle of about 25 degrees to the non-bevel side. This is known as the primary bevel.
Sharpening - or honing - this bevel in effect creates a second bevel, known as the micro or secondary bevel.
There are several different approaches to honing irons, but using a honing guide, like this one, is perhaps the most accurate way of achieving the correct angle for the micro-bevel, consistent across the width of the iron.
A honing guide is a small jig which rides on a roller. The iron is clamped to the jig and the angle at which the iron is to be sharpened is set.
The guide is then rolled backwards and forwards along the sharpening stone with the cutting edge of the iron pressed down onto the stone, producing a secondary bevel at the desired angle.
Step 1 - Check primary bevel
First, check that the iron's primary bevel is angled at 25 degrees to the non-bevelled side of the iron.
If the angle of the primary bevel is incorrect, you may need to grind a new bevel using an electrically-powered grinder, or take your iron to a professional workshop to get this done.
Step 2 - Secure iron in honing guide
Assuming the primary bevel is correct, fasten the iron into the honing guide, with the bevel facing downwards. This usually involves turning a knob to tighten a clamp onto the iron.
Step 3 - Set angle
Now follow the honing guide's instruction leaflet to set the angle of honing to 30 degrees - that is, 5 degrees less acute than the primary angle. This means that the honing will affect only about 0.8mm to 1.6mm (1/32" to 1/16") of the primary bevel - and this is all you need!
Step 4 - Hone iron
Sprinkle some water on the 800 grit stone and begin honing by rolling the honing guide backwards and forwards along the stone while pressing down quite firmly near the cutting edge of the iron.
Step 5 - Camber iron
Some types of planing call for a slightly cambered (rounded) cutting edge. For instance, when smoothing wood faces that are wider than the iron, a slight camber on the cutting edge helps avoid leaving ridges on the surface with the sharp corners of the cutting edge.
Camber the iron slightly while honing by increasing finger pressure on first one side, then the other side of the iron.
Step 6 - Check progress
Check occasionally that you are getting a secondary bevel of a consistent size right across the width of the iron. Gently run a finger across (not along!) the cutting edge to check if it is razor sharp.
Step 7 - Remove burr
A burr is a fine roll-over of metal, in this case from the secondary bevel to the opposite side of the cutting edge, created by the sharpening action.
Remove it by turning the iron over and, holding the non-bevelled side of the blade flat on the stone, sliding the iron along the stone.
Burr is usually quite visible and you might see particles of it on the sharpening stone after it has been removed.
Step 8 - Repeat on higher grade stones
Repeat steps 4 to 7 on the 1000 grit stone, and then on the 1200 grit stone until you have a razor sharp, polished and, if desired, slightly cambered, secondary bevel.