What is a veneer caul and how do I use it? When pressing a veneer, you need to apply even pressure along the entire surface. A caul is used to achieve this. Basically, it is a flat piece of wood that goes on top of the veneer. When the vacuum bag, for instance, presses down, it will press on the caul which will then apply even pressure on the veneer. Make the caul slightly larger than the project you are veneering. If it is too large and the caul overhangs the workpiece by more than ½” on any side, the pressure from the bag will either break the overhanging area, or will put enough pressure causing the caul to bend, causing an area where the caul is no longer applying any pressure on the workpiece.
What is the best material to use for a caul? A good material to use
when making a caul is ¾” melamine particle board. The thickness
gives it strength to withstand any chance of breaking or bending if it overhangs
the project by too much. A melamine surface, as opposed to MDF, is also a
good choice because PVA glues do not stick to melamine. This avoids any chance
of the workpiece sticking to the caul due to glue seepage.
How do I make a caul? Unless every workpiece is the same
shape and size, you should be making a new set of cauls for each project.
Take your caul material, and cut it so that it is slightly larger than the
project you are veneering.
Round the edges and corners of the caul by sanding or routing a smooth curved
profile. What you are trying to accomplish is a soft edge so that the vacuum
bag does not break as it compresses around a sharp corner or edge.
Do I need a caul on the bottom and the top? If you are veneering both sides of the substrate at once, you will need a caul for both sides. Although it is not absolutely necessary to have a caul on the bottom as it will be pressed against the bottom of your press. However, chances are that the bottom of your press will not be flat, it will have a grooved surfaced to allow for air to evacuate. Additionally, a bottom caul, again slightly larger than the workpiece will catch any glue run out, saving you from potentially cleaning the glue from inside the veneer bag.