Ply Prices.

I think any finnish applied to wood makes it slightly stiffer especially two pack materials and Polyester. Fiberglass is the obvious stiffener. Having made flight cases for many years I used many materials including carpet and fiberglass. Diamond board was used extensively at one time, and I would say the had to be the toughest finnish on a ready prepared board. The coating on these boards are resin and fiberglass scrim.
On early 2x15 Concert bins, Where the boards used on the horn section were slotted to aid the bending, I used to fiberglass the backs. This was a real pain as the resin would run every where, and it would take ages to clean up. In the end, the stiffness it provided could be obtained by using more or thicker ply for the horn sections, and of course was quicker to produce. Knowing how to get the best results for a cab in the market place has to balanced with what people are prepared to pay. If it's only for yourself, then you can pull out all the stops and indulge yourself. more wood, concrete, lead, etc. and of course, the support truss.

Thanks for the answer Tony. I haven't tried this myself, except I might just try slapping a coat of varnish on the insides of some cabs I made recently. They're mostly made of 1/2 inch ply and even though they're quite small and well braced, I'd like all the strength I can get.

Where are you on the whole polyurethane vs. PVA debate? I switched to polyurethane a cope of designs ago, and I have to use scre-and-glue to stop the panels coming apart as it sets (don't have the right kind of clamps).

I think poly glue is great but impractical for speaker building. There are many different strength PVA glues and of of course, there's Weld Bond which is what I use to re-cone speakers with. This is probably the strongest normal glue I have come across. The clever bit in production is only doing what is necessary. For years I used cascamite ( not sure about the spelling ) and our fixings have been half staples half screws. Later the glue became PVA. It has to be said, that in spite of all the hi-tech glues available now, to my knowledge, we never had a cabinet fall apart. Some of the PA cabs have now been out on the road for 30 years or more and still going strong.
I think one of the weakest points with the gluing, is not the glue, but the joint. I see so many pictures of cabs that are butt jointed. These type of joints need good glue and lots of screws. Grooving and rebating the panels for fixing adds so much strength to cab.
I have just completed a Fairground system comprising of rectangular boxes. Not one screw was used in construction. Staples only, and good joints of course. I don't think there is any more I can add to this, without sending everyone to sleep.

hi mate

I use 18g brad nails at the moment but want to upgrade to staples.
what size staples do you use and can you recomend a good gun to use?

Cheers, HBL

There are quite a few guns on the market. We used to get ours from Duo fast. If you bought enough staples you would get the gun free. I'll check the spec on the boxes at the factory. Resin coated staples are the ones to use. They have a little more grip than normal staples. I think the ones for putting 18mm ply together are 38mm long. all sizes are available.