I am moderately regularly asked,  at intervals,  about the choice of glues for marine work.

In the course of trawling through some archives earlier today I came across this correspondence on the subject with John Welsford,  which I think may well have originated in a private exchange of emails,  but he was entirely happy for me to reproduce it online provided he was credited as the originator..

For those on this present forum who may not know him, he is based in New Zealand and is an internationally known designer and builder of characterful cruising dinghies.


Oliver

From John Welsford:

Oliver, one of the things that I have had to do for a crust over the years was quality control in factories that make those monster structural gluelam beams you see in school halls and gymnasiums, This gave me access to some very sophisticated testing machinery and the skills to use same.

As a boat designer I frequently get asked about alternatives to epoxy glue, so tested every glue I could get my hands on, as many brands of each type as was available.

Gorilla Glue and Balcotan both tested at around 60% of the strength of Marine Epoxy ( West in this case) and less than 50% of Resorcinol or MUF. The strongest glue of all was white carpenters pva which was a surprise, but to get that strength required exacting fits, careful surface prep, control of moisture content and very high clamping pressure.

Plus its not waterproof.

Back to the Polyurethanes such as Balcotan, none of them came up much better than the figure above, that includes the various cartridge adhesives used in the building trade as well as both the clear and foaming polyurethane adhesives.

My testing results were very consistent with similar tests published in WoodenBoat magazine and the American Fine Woodworking magazine.

John Welsford





My reply to John:

John,

Many thanks for that information, which not only tends to confirm what I found today but also adds considerably to it.

Given that a minority of amateur boat builders and boat repairers urge the merits of PU glues such as Balcotan quite vigorously I think your comments would be well worth putting on the Openboat site, and also – with your permission – I would like to put them onto three other sailing sites …  …    Would you be happy for me to do that, and if so would you prefer to post them yourself  …  or would you prefer me to do so on your behalf?

On the matter of carpenters’ PVA glue, as you rightly say it is not waterproof, although some brands at least offer it in a choice of interior or exterior grade, the latter being water resistant.

Wearing a different hat, as a (now sadly lapsed) amateur violinist and the proud owner of an instrument from the time of Bach (specifically, 1740) I am well aware of the usefulness of glues which are strong but only semi-permanent, for joints which for a variety of reasons may well need to be taken apart and remade at a later date, although they are generally of no use in boatbuilding. I learned the former the hard way, when the finger board of the violin came adrift, so I re-glued it with Araldite, and got seriously raised eyebrows when I mentioned to a professional violinist friend that I had done so; then many years later when I had to take the instrument to a violin maker for a general overhaul, which would involve taking it apart and re-gleuing everything, and resetting the neck and the fingerboard, I told him about this and he said that in the worst scenario he would have to plane it off and fit a new one!





Reply from John:

Hi Oliver, good to hear from you. Thanks for asking and yes, go ahead, just put my name in as the source.

The Americans love PL premium, and Titebond3, the first being a construction adhesive and the second an aliphatic pva. I’d use the second in some areas of a yacht interior, and its supposed to be water resistant but I’d only use it where there was very good ventilation and well above any chance of bilge water.

I know about the glues used by luthiers, have some myself and its surprisingly strong and durable, just needs heat to undo it.  That’s useful as you say, especially for musical instruments.

Yours
John Welsford