Overview Primarily, the sidewalls of ski serve to protect the wood core from moisture damage and impact. 

The most common materials used for sidewall construction are ABS plastic and UHMWPE (P-Tex).  These plastics are ideal in that they are both extremely durable as well as have a high resistance to moisture absorption.  However, sidewalls can also be made from wood or even epoxy.   Each method is detailed below.
 

Making Wood Sidewalls  Wood sidewalls are the easiest to make.  To make wood sidewalls, simply make the ski's core wider than the ski's base (w/ edges) and layup the ski as usual.  After the ski comes out of the press, trim the excess material exposing the wood sidewalls.  Be sure to treat the wood with some sort of wood treatment to prevent moisture damage.

If you plan to use different types of wood for your core, one option to consider is laminating the harder woods near the edge of the ski.  Harder woods are usually more durable.
 

Making Epoxy Sidewalls  Epoxy sidewalls are the easiest to make, and they can can be made into different colors, whereas ABS/P-Tex plastic usually only come in black or white.  Colored epoxy is achieved by using special pigments designed specifically for epoxy.

There are several methods for making epoxy sidewalls.  One method (we call the Moat method) is to form the epoxy around the blank wood core, then profile the core with the attached epoxy.  Another method (referred to as the Trough method) forms the epoxy sidewalls after the wood core has been profiled.  Both of these techniques are outlined below beginning with the Moat method.

Moat Method

Step 1:  Router Out A Channel in the Core  To make epoxy sidewalls, begin by using a router with a template to cut out a moat, or channel, that surrounds the ski's perimeter within the blank core.  The template should be made so that the center of the router bit (1/2"-1" diameter) will align with the ski's metal edge.  The diameter of your router bit will determine the size of the channel and in effect the width of the sidewall.   So choose the router bit accordingly.  Typically you would use the a double-edged straight router bit (1/2"-1" in diameter) along with an appropriate sized bearing to follow the template.  An example of such a setup is shown below:

The key is to ensure that there will be enough sidewall material to protect the core and also to cover the metal edges.  When routering, make sure the bit doesn't cut through the entire depth of the core.   The main objective is to create a moat that will hold the epoxy as it cures.  For example, the picture below illustrates the process:

Step 2:  Fill Channel with Epoxy  After the channel has been routed, simply feel the trough with epoxy, then wait until it cures before profiling the core. 

Notice that this method creates both the sidewalls and tip spacers.

Trough Method

Step 1:  Attach Plexi-Glass to Profiled Core  Cut two rectangular pieces of plexi-glass that are as long as your core and approximately 3-4 inches wide.  With your profiled core clamped vertically so that one edge is facing upwards, secure the plexi-glass to the profiled core using strong double-sided duct tape along with clamps.  A trough should be created as shown below:

Try to make sure the core is as level as possible.  If not, epoxy will build up entirely on one side.

Step 2:  Pour Epoxy  Mix up your epoxy (the same kind used during layup) and add any color pigments at this stage.  Carefully pour the epoxy into the trough.  Check for any leaks.

Keep adding epoxy until you have the desired width of sidewall. 

Step 3:  Finishing  After the epoxy has completely cured, carefully remove the plexi-glass.  Sand the surface of the epoxy with some rough grit sandpaper so that it bonds well during layup.  Keep in mind that the above steps have to be repeated three more times for the remaining sidewalls.
 

Making ABS or P-Tex Sidewalls  Making ABS/P-Tex sidewalls requires the most work out of the three methods, however, the results are much stronger than those obtained by using wood or epoxy.  The outline below demonstrates how ABS/P-Tex sidewalls are attached to a wood core.  Note that the method is the same whether you choose to use ABS or P-Tex.

Step 1:  Trim Wood Core to Shape  Before you can attach the sidewalls you must cut the blank (not profiled) wood core to shape using a band saw or jig saw. 

When cutting your core to shape keep in mind that you must leave enough room for the plastic sidewall material.   The actual width of the sidewall is up to you to determine but typical widths range from 1/4" to 3/4".  The following picture shows the wood core on top of the base material (w/ edges attached). 

Notice that there is about 1/4" of room for the sidewalls to extend into the ski.  When attached, the sidewalls will actually cover the edges and overhang them (the excess material is trimmed off in the finishing process).  Also note that the tip of the core is only roughly cut.  This is because this portion will undergo additional trimming when attaching the tipspacers so a precise cut is not necessary at this point.

Step 2:  Rip Plastic Into Strips  After the core has been cut, the plastic needs to be prepared.  You usually find thick ABS/P-Tex material in large sheet forms (1/2" x 48" x 96").  These sheets need be ripped into strips using a table saw or any other saw you are comfortable with.   For a pair of skis, you will need four strips (two for each side of a ski).   The width of the sidewalls strips vary depending on your ski design.  Just make sure that you have enough to overhang the ski's edge by approximately 1/4" or greater.

Step 3:  Flame Treat the Strips  Most manufacturers of ABS or P-Tex recommend that you flame treat the plastic before it is bonded to the core.   This is done by simply using a torch and quickly passing the flame over the plastic. 

Only let the torch stay in one location long enough just to slightly darken the plastic material as shown below. 

You only need to flame treat the side that you intend to bond to the wood core.  The other sides (top-side and base-side) will be flame treated after the core has been profiled.

Step 4:  Attach Sidewalls to Core  Mix up a small amount of epoxy (the same kind that you will use for layup or something similar).   Use a small, disposable brush to apply the epoxy to the vertical sides of the wood core.  Then apply another generous amount of epoxy to the flame treated sides of the sidewalls.   Arrange the sidewalls and core together and secure into place with some clamps as shown.

After the epoxy has completely dried, remove clamps and proceed to the next step of profiling of the core. 

Note that it is also possible to make a full wrap around the core with the ABS/P-Tex to create both the sidewalls and tipspacers at once.  However, this requires a little more work and the plastic will have a greater tendency to "blow out" during skiing.