From Joe Grubaugh, it's the
No-mess Plaster Cast
© Frank Ford, 7/15/98; Photos by FF, 7/14/98

From time to time, I have to do some major surgery on the top or back of a carved guitar or mandolin. In these cases, it's really handy to make a plaster cast of the area on which I'm working. That way, I have an absolutely solid work board to support the delicate carved part while I clamp or work on it.

The conventional way to make such a cast usually involves making a shallow wooden frame or pan for the plaster, then covering the instrument or part with foil or plastic and squishing it into the plaster to make a mold.

Joe Grubaugh, violin maker from Petaluma, California, came up with the simplest and neatest system I've seen for avoiding all the plaster mess.

Here's how it works:

First I'll pour my plaster of Paris mix right into a zip lock plastic bag:

Next, I "burp" out the air from the bag and seal it up:

A quick wipe with a wet towel and I have no more plaster mess on me, my bench or anywhere. How neat!

Now I can simply lay the bag right on the surface I want to match and it will conform to every contour:

I'm placing a scrap board on top and weighting it to flatten out that area.

I stay close by to monitor the plaster as it sets. The plaster really heats up as it begins to harden, so I want to remove it as soon as it becomes stiff and begins to warm. I need to get it right off of there as soon as it gets warm because the heat and will certainly damage the instrument's finish. It's no problem though, because the plaster is quite stiff when it starts to warm up.

Once the plaster stiffens, I remove it from the instrument and cut the bag off. Then I can place my scrap board back in place on the plaster with some glue and set the whole business aside to dry overnight. There's a huge amount of water in the plaster, so it can dry for quite a few days, but I can work with it the next day if I protect the instrument's finish with plastic when I clamp it.

After the plaster has fully hardened and my board is glued to the back, I simply run the whole assembly through my bandsaw to trim it to the size clamping caul I need:

It fits the uneven contour of this old mandolin perfectly so I can clamp a patch inside without deforming the top even a tiny bit.

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