Thoughts on Gluing
wood by Tony Sykes
There is so much that could be said on the subject I could fill book, so must restrict myself to the one glue that I am most conversant with, RESORCINOL especially Aerodux 500 and 185, manufactured by Ciba-Geigy, though I have seen just as reliable results with CASCOPHEN, by Bordens which has the advantage of being available in small quantities from chandlers.
Aerodux 500 and it's hardener 501 makes a thick treacly purple stodge and the colder it is the stodgier it becomes. 185 is similar except the hardener is a white powder 155. 500 is supplied Fast (f), Slow (s), and Medium (m). I used 500(m). If stored in a cool place it has a shelf life of at least a year and in practice, longer I have found this most useful and convenient, mixing equal parts together.
185 has a shelf life of over two years, though less simple to use as the powder gets everywhere, but is Just as strong for; all that.
To obtain the very best results the moisture content should be in the 12-15% range. Timber bought as kiln dried and stored under cover will present no problem although RESORCINOL glues are more tolerant than most and will work well with 25% with only a 10-15% loss of strength.' Epoxy glues would almost certainly fail if used with wet timber.
Perhaps as important as the glue is the preparation the faying surfaces, especially for oily timbers, Teak, Iroko etc. Timber direct from machining have a slightly glazed surface and the surface fibres are compressed by rollers etc, far from ideal and it is essential that the surfaces to be joined should be ridded of this unwanted smoothness. For this I favour the old fashioned smoothing plane which looks like a wooden smoothing plane except the iron is vertical, the iron has fine vee incisions cut vertically into it's face and when sharpened in the usual way produces cutting edge of sharp points. It has no back iron, mine dates from tender youth and veneering by hand with enormous cast iron pots of scalding hot Scotch glue. The use of this tool produce's a surface with a faying area increased by at least 200%, coarse glass paper hack-saw blades etc can- of course be used, albeit not as effectively. Plywood must be, treated in the same manner oily timbers benefit from degreasing and I used Tricocthylene (ch) purchased from a friendly chemist. Two 3 litre flagons saw the boat launched. In practice, after toothing the faying surface was dusted with a bristle brush, then wiped over with a clean rag soaked in 'tric', this became a habit whatever timber I was using, as it not only degreased but dusted. As important as preparation, is working in the correct temperature and that means, within reason, as warm as possible, certainly not below 60F and 7O-8O F I found ideal. In summer in a polythene covered shelter the temperature was often over 90F, it was more a matter of working at breakneck speed, pressure must be applied before the glue gels. Even in the depths of darkest winter it is possible to achieve local conditions of high temperature. For example, when laminating beams, a heater was placed under the jig which was supported on two trestles, a mild steel plate on top to act as a diffuser and then an insulation of mineral wool sealed in polythene, was draped over the work after the cramping was complete.
At higher temperatures the glue becomes much less viscous, easier to apply and seems to penetrate the timber instead of laying on the surface, also a better glue line results, for the best results both surfaces of the joint should be glued. I used Turkshead glue brushes and 2" paint brushes for larger areas. Use good quality brushes that are not forever shedding hairs. Surfaces once glued should not be exposed to the air for any length of time and in practice I found the only operation that presented any difficulty in this matter was when planking with large sheets of ply and my son was press-ganged into assisting, with one gluing the framing, the other the ply, the surfaces were brought into contact in a matter of minutes. Once in contact one has a reasonable time for pressure to be applied, either by clamp, screw or bolt, varying from hours at 60'F to an hour at 90'F, but obviously the sooner the better. Do not over cramp, it's possible to starve a joint this way. Just bring the surfaces firmly into contact.
I found that the removal of surplus glue was best carried out when the glue had reached a jelly like stage, just before it went off, it comes off readily and doesn't stick to anything. If one attempts to scrape off the surplus whilst still runny, it can be a very messy business indeed, rather like model making with black treacle. On the other hand if the glue sets hard it will prove very arduous to remove it and from awkward places almost impossible.
There are common sense precautions to be taken whilst using Resorcinol glues, though the literature supplied lays down very stringent and daunting prohibitions, aimed at the industrial user, having much of the pessimistic foreboding of an Admiralty Pilot. I found that by using cheap polythene gloves, hands could be kept fairly clean, however as the glue sets very quickly on warm hands I sometimes found it necessary to resort to methylated spirit to remove it, despite the manufacturers advise to the contrary, I suffered no harm. The glue, especially when the weather is warm, gives off pungent and fairly unpleasant fumes and if one became hooked on sniffing it, I feel it could do you no good, if you are particularly sensitive it would be sensible to use an industrial mask, but with spasmodic use of the amateur boat builder no ill effects are experienced. Brushes were suspended in meths and washed with soap and water. The only other adhesive for wood I used was a tiny quantity of Cascamite which was found useful for odd jobs but in general Urea Formaldehyde glues, Aerolite, Cascamite, are not satisfactory for oily timbers where maximum structural strength is required. In conclusion I can report that my boat has been sailed hard and often and during the most horrific ten minutes of my life, suffered being bounced with crashing force on the concrete hard sands of the North Buxey, with never a sign of glue failure, in fact any modification I have carried out has meant the virtual destruction of the timber involved. This of course could prove a major drawback to anyone addicted to constant alterations to their boat.
A footnote to this page, as many have asked where can you buy this stuff. Sadly I have not seen it for sale for years, but as all waterproof ply, including 1088 marine ply is glued with it, clearly it is still made. Anyone know where you can still buy less than a tanker full?
Today we use epoxy of course......
December 2009, from Darren... 'most aviation suppliers sell it. Try LAS aerospace ltd or sky-craft.co.uk.'
Thanks for the info Darren.
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