When using the guitar mold, it is useful to have a method of holding the sides against the mold without having to use several clamps. A side spreader can be made from a piece of MDF or scrap that will hold the sides against the mold tightly, and be easy to remove when needed.
This simple spreader works by using pressure to hold the two pieces together as they also hold the sides in place in the mold. Since most guitar sides will be very close to the same thickness, only one needs to be made for each mold. Also, since the piece being made can be from scrap, the cost is very low too.
Start by having a bent set of sides in the mold, with them held tightly against the form by several strong clamps. It is very important to get them completely flush against the sides because this will be used to measure for the spreader. If the sides are not completely flat, the spreader will be made too short, and every guitar made from this setup will have slightly crooked sides as well.
Once the mold and a set of bent sides are well clamped, set the whole unit on top of
a board that is 4/4 thick, 3" wide, and 14" long. The board should show through the bottom of the mold and be positioned at the waist.
The mold and the board should be laid out as in the diagram above, with the board below the mold, and evenly placed at the waist. The clamps and sides should still be there, though they are not drawn into the diagram.
Trace the outline of the waist onto the piece of wood with a sharp pencil, as close to the sides as possible. Once the marks have been made, the piece can be removed and the mold set to the side.
On the band saw, cut just outside the outline drawn on the spreaders, and refine the edge with a belt sander or curved sanding block.
Draw a center line as in the diagram above, and cut it out with a thin kerfed saw blade on the band saw or table saw. Try not to remove more wood than necessary, since this will affect the fit.
There needs to be a tiny bit of wiggle room in order for this spreader to work properly, which means that sawing out a thin line to remove a little material is actually an important part of the building process. If a thin kerf blade is not available in the shop, simply make the vertical cut first before placing the board under the mold to mark the waist. Make sure the two pieces are pressed together tightly when doing it this way, and mark the waist profiles on each side.
With the boards marked this way, cut each one individually on the band saw the
same way as before, and sand down to the line accurately. Either way this is done, proceed to the next step once the center line has been cut.
In order to be able to separate the pieces for removal from the guitar once they are no longer needed, a small notch needs to be created. This will allow the pieces to be pressed at the center, causing the pressure joint in the middle to fail, and the pieces to simply fall down.
Once they have fallen, they can be removed from the guitar.
The reason these pieces are made only 3" wide is so they can be pulled from the guitar soundhole after the entire body has been glued up. If they are any larger, they will not be able to be pulled out of the soundhole, and a large rattle will have been made instead of a guitar.
These spreaders are meant to be left inside the guitar the entire time it is in the mold, and pulled out through the soundhole after the plates have been glued into place. If the soundhole on the guitar being worked on is smaller than the width of the spreaders, make them thinner with a quick run through the band saw or table saw. Also, if a larger soundhole is being made, or an oval soundhole, feel free to use a wider piece for the spreader, which will distribute the force better.
Lastly, another spreader can be made in the same way as the first one, but for the top and bottom of the guitar. This should be made form a longer and thinner piece, so it can be removed from the soundhole as well. Make sure the end blocks are already in place before measuring out for this spreader, because it will be too long otherwise.
When making the guitar body in the mold, place the side spreaders in as one piece from the top, positioning them at the waist, about half way up the side of the mold. Glue the head and tail block in place, and then add the vertical spreader once the glue has dried and the clamps are removed. With both spreaders in place, the kerfing and the plates can be glued on in order and left to dry overnight. The following morning, press on the centers of each spreader and they will collapse under their own pressure. Remove them from the soundhole, and proceed to the next step on the body.
Only one set of spreaders should need to be made for each mold, since the sides and thicknesses of the head/tail blocks will typically be very similar. When using the spreaders, insert them carefully so not to crack or crush the sides. A snug fit is a good thing, however it should not be frightening to insert these pieces. If they are hard to place, simply hand sand one of the spreaders at the center joint using a block and sandpaper, not a belt sander. Usually a few strokes with the sanding block make it easier to get into position.