There has been a lot of confusion on this particular subject over the years and basically I have seen both success and failure with both untreated and treated timbers using epoxy glues over the years. As a marine surveyor and owner of a site that sells boat plans, I have been asked about this on many occasions and have always answered….” Well, I really don’t know, I have no evidence one way or the other, so maybe better not to!”
However I have always been curious about it and decided to look into it further. The reasons for this are many and varied but most people who do agree with the process seem to agree that if the treated timber is very dry it will have a greater chance of success and be more likely to bond successfully. I say that, basically, as an initial observation.
Plywoods that are ‘hot press manufactured’ do not usually have the same humidity content as solid lumber. This is partly due to the high temperatures used in the manufacturing processes. The high temperatures cause the plywood to assume a lower moisture content than the surrounding relative humidity. Then again, most of these are rarely, if ever, subject to the chemical treatment processes anyway, so probably don’t count in this context.
However, it would appear that due to high turnover of timber in populated city areas, a high demand and high turnover for timber and it would seem that many chain stores like Bunnings here in Australia use a lot of plantation type timbers that are still very wet from the preserving treatment and in my opinion, not dry enough for sale. It does not help matters that the timbers are bundled in plastic and this certainly helps to retain any moisture present already. I tend to avoid these like the plague as epoxy glue will certainly refuse to stick to these timbers as the excess humidity will certainly prevent a decent mechanical bond between wood and glue.
Well, just how dry does timber have to be whether it be treated or untreated? Well, in a living tree there are huge amounts of water and sap inside that allow to tree to live and grow. Once cut and used as lumber the ‘green’ or ‘wet’ timber starts to dry out. As mentioned, commercial pressures and artificial and forced kiln drying does not allow the hollow cellulose tubes that once allowed the sap to flow, to shrink at a proper rate and the moisture content , often far too much, is still retained in the timber when it is put out for sale prematurely.
Most timber will, if stored properly, maintain an equilibrium with the surrounding air humidity so wood stored and kept in humid tropical areas will probably be much wetter than those, say, stored in a drier climate elsewhere. One American standard in interestingly enough, for kiln dried timber is around 19 % whereas another in a different state is as low as 15%! Personally I would be loathe to attempt to glue with epoxy at 19 % and marginally at 15%! Some timber for furniture and boat building is satisfactory at around 10 to 12% but some very fussy cabinetmakers will demand 7 to 10%!
There are some other types of water based glues available that seem to work on high humidity timbers and there is one British made glue called ‘Polyproof’ and is deemed waterproof completely but it appears quite hard to obtain here in Australia and also leaves a red glue line behind. Once again, it is unlikely that it will have the strength of epoxy but may do well in other applications if boatbuilding.
However, once again mindful of stress requirements, there is one glue that positively thrives on the damp. In fact, the glue needs the damp in order to allow it to cure. The glue is Purbond, a single pack expanding polyurethane glue available from Boatcraft Pacific in most major cities.
However, I am unsure if treated timber will work 100 percent with that particular glue …call them up and see! One other factor is that there are many varying types of timber preservation treatment chemicals and the bonding ability of these differing chemicals may well vary in order allow epoxy based glues to work as intended.
For example, oily type preservatives are harder to bond. Certainly but how would you recognise them? Strong creosote and pentachlorophenol based treatments should be steered well away from as epoxy certainly doesn’t like them at all, especially when combined with moisture. On the other hand, many of the better quality constructional type adhesives are reputed to be able to withstand fairly moisture laden timbers, up to around 15 to 20%. In fact, surprisingly enough, there are some fairly positive reports about these glues on many boatbuilding forum sites.
Another factor to take into account with treated timbers is the fact that many acidic salts that are present in the chemicals that prevent natural drying out rates to occur. Added to that that many processing plants store the treated wet timbers outside where the noxious fumes are able to disperse to the atmosphere and are subject to weather conditions.
There is of course to be taken into account, the actual direct chemical cocktail reaction that happens when epoxy and its internal constituents are placed into direct contact with chemicals such as arsenic and phenol, for example! There is every reason to expect that the resulting combinations will repel rather than attract!
A couple of tips if you do decide to bond treated timber with epoxy: Do a practise run first with the glue and timber of your choice. Give the joint a hard test time after curing! Sand or lightly plane the surfaces first before gluing…
Always wear protective clothing and dust masks when cutting or sanding treated timber. It’s job is to be fairly toxic to insects and humans too can be very affected. Never underestimate the power of these strong chemicals!
Another alternative to treated timber is high quality untreated cypress pine. It is hard, water resistant due to being rather oily and lasts well. Finally, try to use stainless steel screws in treated timber as plain old steel screws tend to melt quite quickly in the chemicals and sap!
Well. why glue with epoxy on treated timbers at all? Yes, a very good question I must admit!… During the writing of this article I have done a fair bit of investigation and really although I have found some interesting data I haven’t found too much that has endeared me to the processes of using epoxy on treated timbers overall…this is for marine purposes, I might add.
However, there is a wealth of information out there that does suggest that many of the modern glues and more especially, many resorcinol glues can do a pretty effective job when it comes to other applications on dry land …I think that until thinking outside the square becomes totally illegal, a healthy curiosity still has a place in the world!