cloth tape side reinforcement

[Kary Karahadian]

I've read of using poly-cloth strips as side reinforcements that are placed first, then the linings over that. Doesn't that create a slight gap between the lining and the side adjacent to the tape that could cause peeling of the lining? Is this a non-issue? i've always built with laminated sides (until this build) and never worried about reinforcements.

[Alan Carruth]

I've been using that system with cotton-polyester bias selvage tape for decades. Nylon tapes are stronger material, but don't stick to the glue well. I found through some experimentation that hot hide glue sticks a bit better than Titebond, and is easier to use anyway, so that's what I put the tapes on with. I also use HHG for the liners. If anything, they stick better to the tape than the wood! There's no problem with gaps at the tape edges; the tapes are only about .005" thick, which is not too bad for a glue line. Since exposure to air can break down HHG I shellac over the tapes after putting the liners on. We'll find out in 75 years or so whether that works.

[Kary Karahadian]

Thank you Alan. I didn't realize that cotton-poly tape was so thin.

[Alan Carruth]

The bias tape comes folded over on itself; you have to iron it out flat to use it as side tape.

[John LaTorre]

On my next build, I might try what sailmakers call "insignia cloth" which is a thin polyester cloth with an adhesive back. It's used for cutting out the letters and logos on sailboat sails, and it sticks to just about anything and is hell to take off. I happen to have some left over from my sailmaking days.

[Alan Carruth]

Back in the 60's iirc, Martin started using self-adhesive side tapes. The material was a lot like the adhesive tape they used to use for bandages; cloth of some sort with a latex-y adhesive in a rather thick layer, so that it would stay flexible and still adhere. They didn't work out well, for two reasons. First: the adhesive would dry out and come loose from the sides. Since the tapes were put on before the liners (as they should be) the ends would stay down and leave the middle flapping. The adhesive was still sticky, so they just got fuzzy on the stickum side, and on the wood where they had been stuck down. Second: tapes work by holding the wood together, preventing cracks from getting started. To do this the adhesive layer has to be very thin and hard, so that there's no relative motion allowed between the tape and the wood. Martin's self-adhesive tapes, with their thick layer of soft adhesive, didn't do that. When I compared the strength of hide glue and Titebond as tape adhesive, the hide glue turned out to be a bit better; possibly because it's not as flexible?

Back in the 80s (?) I had a customer who used to tour with a Martin D-28 from the adhesive tape era. Whenever he got back from the road he'd be by to get another side crack fixed. After a few rounds of that I removed all the self adhesive tapes, and scraped up the stickum as best I could, working through the hole. Then I put on new tapes of linen cloth in between the areas of stickum, running the ends up onto the liners. He never had that problem after that, which allowed me to concentrate on the top cracks he used to get too.

Bottom line: the devil is in the details... I would suspect that anything that sticks to a sail in use would have adhesive that is too flexible for this use, but I'm not familiar with the material. OTOH, you could probably do some colorful designs that way.

[John LaTorre]

I agree that linen soaked in hide glue would probably be the best solution. But I'm going to use the insignia cloth on a test piece, just to see.

[Alan Carruth]

When I did my experiment I made up 'coupons'; slices of wood from broken sides that were about 1-1/2" along the grain and 4" or so across it, and around 2 mm thick; normal side thickness. I put the tapes on in the usual manner, with the different glues I wanted to test, and shellaced them over as I normally do. I made up a rig that had a long lever arm to put force on them to see how much it would take to break them. The coupons were set on a pair of dowels about 2" apart, tape side down. The dowels were along the grain. The lever had a dowel on it that came down just in between the two supporting dowels. I then slid a can full of marbles slowly along the lever until something broke. By knowing the weight of the can and the distance out, plus the 'tare' of the lever itself (which I got using a scale) I could figure out the force it took to break the coupon. There were, of course, also coupons of the various woods that had no tapes on them for comparison. The differences seemed to be fairly clear, although, of courser, one could always wish for a bigger sample...

[John LaTorre]

When I did my experiment I made up 'coupons'; slices of wood from broken sides that were about 1-1/2" along the grain and 4" or so across it, and around 2 mm thick; normal side thickness.

Thanks for the write-up, Alan. Your methodology seems sound to me. But what were the results of the tests?

[Alan Carruth]

"But what were the results of the tests?"

That was summarized in an earlier post, sorry. For completeness:

I tried both nylon twill tape, similar in weight and look to the linen tape Martin used to use, and also the cotton/poly selvege tape. I tried Titebond and hot hide glue as adhesives, and all samples were shellaced over. I used several different kinds and thicknesses of side wood, which had a wide range of strengths. Basically, any sort of tape with either glue increased the force it took to break the samples by 40%-60%, as compared with 'bare' ones, with larger effect on the weaker woods or thinner samples. There was a small but consistent difference due to the glue used, with hide glue being a bit stronger. The nylon tapes tended to fail along the glue line: the tapes themselves did not break, but simply peeled away from the wood. The cotton-poly tapes were far more likely to stay glued down and break where the load was. Keep in mind that in this test the tape was on the tension side. Skeptics of this method often point out that old side tapes they see have broken where the sides are cracked, inferring that the tapes did no good. There's no way to know after the fact how much force it took to crack a side, but my tests suggest that the tapes significantly improve the strength. I have also seen instances where an impact crack that starts between tapes tends to stop when it reaches one: I'd much rather fix a 3" long and tight side crack than one that runs all the way from the neck to the tail...