How to flatten the soles of metal
The sole of a plane, whether metal or wooden, needs to be perfectly flat, otherwise the plane cannot be held flat on the wood and the cutting will be uneven. The only exceptions are some specialist planes - those for cutting circles and other curved shapes.
How to check the sole of your plane
If the sole is found to be uneven when you ‘sight’ along it (look along the sole from toe to heel), or place the edge of a ruler or try square along it, you will need to follow a procedure known as "lapping the sole".
Another way of checking is to place the plane sole-down on a perfectly flat, hard surface such as plate glass, and see if it’s possible to rock the plane.
The above checks are ones you might be able to try before actually buying a plane.
Yet another way - which you can probably only do with planes you already have - is to mark the sole with a marker pen and run the plane across abrasive paper on a flat surface
If the ink wears off evenly, you have a flat sole. If high and low areas are revealed (by the removal or retention of ink respectively), you need to lap the sole.
Lapping the sole of a metal plane
You will need:
Step 1 - Stick abrasive paper to flat surface
Tape or glue two pieces of abrasive paper – one 240 grit, the other 120 grit – to a flat surface. If glueing, make sure the glue is spread perfectly evenly and there are no ridges in the paper.
Step 2 - Mark sole of plane
With a fairly broad-tipped marker pen, mark some lines across the sole of the plane.
Step 3 - Run sole over paper
With the blade retracted, run the sole of the plane over the rougher, 120 grit abrasive paper, backwards and forwards, a dozen or so times. Put some downward pressure on the plane, trying to equalise the pressure along the sole.
Step 4 - Check progress
Take a look at the sole – you should see that some of the ink marking has been taken off by the abrasive paper. The "high spots" of the sole are the parts where the ink has been removed, and the "low spots" are the parts where the ink remains.
Step 5 - Continue to flatten
Continue to flatten the sole by running it over the 120 grit abrasive paper until all the ink markings have disappeared and the sole is shiny.
Step 6 - Smooth sole
With the the sole now flattened, any scratches left by the rougher paper can be smoothed away by running the plane over the finer, 240 grit paper.
Lapping the sole of a wooden plane
It's usually easier to lap the sole of a wooden plane than a metal one as wooden soles, although usually made of the hardest of hardwoods, yield more readily to abrasive material.
You will need:
There's no need to mark the sole of a wooden plane with a marker pen as you do with metal ones.
The scratch pattern made by the sandpaper will readily reveal any high and low spots on the sole.
Step 1 - Tape or stick sandpaper
Tape or stick the sandpaper to your flat surface. The sandpaper should be at least the length of the plane.
Step 2 - Run plane over sandpaper
With the iron retracted, run the sole of the plane just once over the sandpaper, pressing down and maintaining firm contact throughout.
Step 3 - Check sole
Take a look at the sole. The scratch pattern left by the sandpaper should reveal any high and low areas of the sole – the high points scratched and the lows left untouched.
Step 4 - Remove high points
If there are any high points, remove them by running the sole backwards and forwards repeatedly over the sandpaper until the whole of the sole has been affected by the sandpaper.
At this point, your sole is well and truly flattened!
Step 5 - Smooth finish
If the sole looks scratched rather than smooth, you can finish off by running it across higher-grit sandpaper – somewhere between 220 and 400.