Monday, 14 October 2013

Evaluation: Stewmac Shielding Paint

So, how many coats of Stewmac Shielding Paint does it take to create a decent shield for your guitar cavities? Good question. Stewmac say to use at least 3 coats of paint, and there's a decent number of people leaving glowing reviews of the product on their web-site. Well, fair enough. But personally, I have my doubts. Hey, all props to Stewmac - their products are of a consistently high quality, a lot of thought goes into them, and many world-class luthiers swear by them.

However, the way I like to wire my guitars is to chain the cavity as part of the ground wire from the bridge to the output. For this to work effectively, a very low resistance is required for this ground connection, and that includes the resistance introduced by the cavity shielding.

So I set up a little experiment to see the effect of incrementally building up layers of shielding paint on the resistance of the shield. Resistance measurements were taken on a square of untreated pine, using a multimeter with probes separated by a distance of approximately 8cm.

The results are given below. As you can see, the resistance decreases as the number of coats increases (as expected). However what is interesting is that the decrease is logarithmic, with each coat having less and less of a cumulative effect.

Remember, this is over a distance of just 8cm. This is similar to the diameter of a Les Paul control cavity, but nowhere near the maximum diameter of the Stratocaster cavity. With the cavity shield wired in series to my ground wire, I would expect a resistance of less than an Ohm anywhere along the ground. Unfortunately (at least for my purposes) the Stewmac paint does not seem to fit the bill. When shielding my current Stratocaster project, looks like I'll stick with adhesive copper foil.