Thread: Advice needed--epoxy not bonding to very dense wood

If the joint design permits, you might enhance the connection with mechanical fastening, such as dowels or biscuits. The dowel holes could have rough interiors, and the "dowels" themselves could even be all-thread steel. BTDT. The mating surfaces can still be sanded smooth for intimate contact. Picture frame miters are often made this way. If you make the holes slightly oversize, alignment will be less difficult.

Cheers,
Joe

Don't thin the epoxy - all that does is weaken it.

You only need to solvent wipe if it's an oily type of timber. Don't overfinish the mating surfaces - a quick sand with no more than 80-100 grit - epoxy relies on a mechanical bond, not a chemical one.

To get the epoxy to sink in (well, as much as possible, which isn't actually all that much) heat the timber (leave it in the sun for an hour or two), spread the epoxy and clamp it up. Don't clamp too hard - you don't want to squeeze the epoxy out, it's a gap filling adhesive after all.

As the timber cools down it will suck the epoxy in, ever so slightly.

Thanks for the replies. I guess I wasn't all that clear. I don't use epoxy on the joint, and I most definitely do not fine sand the joint. The epoxy is used as the finish coat, after the piece has already been glued together with Titebond III. As for doweling to strengthen the joint, yes, I'm just getting into doweling which has been quite a pain to figure out since the didges I'm carving are curved with hardly any square surface on which to attach a jig. For these difficult and dense woods, though, I'm finding doweling to be essential if I want glue-up to go off without a hitch. Titebond III is supposed to have a longer open assembly time, but it's never long enough. I'd like to try something with a longer time. I've read about all kinds to try and think the Lee Valley 2002 formula would be worth a try. It would be nice if I could use epoxy to glue the joint since the squeeze out on the inside of the didgeridoo would blend in with the epoxy finish coat that later goes on the inside. (Cleaning up the TBIII squeeze out from the inside is a major pain.) Problem is, I understand that with epoxy glue you have to have a thicker layer of the glue between the joint. I want an invisible joint. It's taken a lot of disasters to get to the point where my joints are disappearing and I don't want to backtrack by using epoxy to glue the joint. Is there some way around this, or is the thickness required not so much as I am thinking?

Anyway, back to the original topic... after posting I did some more reading and learned, as Master Splinter said, that thinning epoxy with solvent is not a good idea. I'm going to use the recommended technique and heat the wood a little before application. Now that we're clear that I was talking about the finish coat on the outside of the piece and not the joint itself, could someone tell me whether sanding the wood down to #220 grit before applying the epoxy is likely to be a problem?

One last thing...I think it was raining out when I applied epoxy to that piece that ended up with a weak bond. I guess this could have been part of the problem

With epoxy joins you can get an invisible glue line; just avoid going to the extreme and clamping the hell out of things - you actually need epoxy in the join line, so an overfinished surface can be more of a problem than an asset if you like to use the clamps as more of a test of of the compressive strength of timber rather than an aid to maintaining position while the glue sets. If you want long working time, don't heat the timber prior - heat speeds up the setting reaction.

You can also use a bit of sanding dust or tinter in the epoxy; I've found that a tiny bit of red and black Feast Watson tinter in epoxy makes a good match for sap runs, so my occasionally over-casual joint prep effectively disappears.

Bote-Cote make a thinning additive (TRPDA), but I don't think getting the epoxy thinner is what you need. If using epoxy as a finish, don't sand the timber to more than 200 grit, and apply subsequent coats while it is still 'green'. Do a few tests on scrap and see what gives you the finish you are after.

Thanks for all the useful info. I've definitely over-clamped in the past. After the two halves of the didge log are hollowed out, if they remain apart for too long (especially without being sealed) they start to warp away from each other and curl inward so that the joint doesn't fit nice and flat. to compensate, I've clamped really tight, not realizing that I was starving the joint and that in the long term the stress on the wood might overwhelm the glue bond and/or cause glue creep. For the piece I'm working on now and in the future, I plan to keep the wood sealed at all times while working the inside. Although I've done this a couple times in the past--sealing the freshly cut log after splitting it in the bandsaw and then sealing the inside as soon as carved out--but there was still warping. This was dry wood.

These pieces have formulated acoustics (a la Computer Aided Didge Sound Design, created by Frank Geipel in Germany) which require precise internal dimensions to get it right, so it takes a few days to get the inside circumferences just right before glue-up. By the time I've got the dimensions carved just right the warping is in full swing. If it was just a traditional free-form didge there would be no warping problem as I could have it cut and glued back together in the same day. So I've really been looking for a way to deal with the warping and not have so much anxiety about the pieces remaining apart for too long. The warping not only makes for a bad joint and stressed wood if over-clamped but the gap in the joint (on the inside wall of the instrument) undoes all my tedious efforts to get the millimeter-precise circumferences for acoustics.

I know I'm making this into a long reply, but I've really had a lot of questions as the piece I'm working on has kind of been a disaster as far as the warped joint. Epoxy would fill in the gap on the inside of the joint, but it's too beautiful a log and I've put too much work into the acoustics to glue it up with the warping. First I tried steaming the pieces to get them back in line with each other, but this only made things worse as some parts of the joint started to curl outward instead of in. As it's dried, most of the outward curling has vanished. At this point I'm working the inside with grooves, epoxy filler and clamps to get the joint to sit flat again (which means I'm going to have to go and check all my circumferences again for acoustics purposes b/c they will surely have changed). The dowels are also going to help with the alignment.

Not sure about tinting the epoxy for the joint. Parts of the log are heartwood (dark reddish brown) and parts are sapwood (closer to white) so it might stand out badly in some places, but I"ll keep that in mind and maybe do some experimenting with it.

I'm really excited to hear you say that I can get an invisible joint with epoxy. I've been wanting to switch to epoxy for a long time. It would make glue up and inside finish coating a much easier process. Just to think, I could do glue up and finish coat the inside at the same time! It does worry me though because I still don't trust my epoxying skills. One of these days I'm going to find some consistency about it. I guess as you say the best thing is to test it out first.

On that last piece I mentioned (where the epoxy finish coat easily peels off the fine-sanded wood) I did as you said and applied all subsequent coats while the previous coat was green. From everything said, I think the main problems were 1) using solvent on the first coat, 2) epoxying while it was humid from rain and 3) sanding the wood too fine.

Again, thanks for the advice. And now back to the grind....

Originally Posted by didgemaker

Thanks for all the useful info. I've definitely over-clamped in the past. After the two halves of the didge log are hollowed out, if they remain apart for too long (especially without being sealed) they start to warp away from each other and curl inward so that the joint doesn't fit nice and flat. to compensate, I've clamped really tight, not realizing that I was starving the joint and that in the long term the stress on the wood might overwhelm the glue bond and/or cause glue creep. For the piece I'm working on now and in the future, I plan to keep the wood sealed at all times while working the inside. Although I've done this a couple times in the past--sealing the freshly cut log after splitting it in the bandsaw and then sealing the inside as soon as carved out--but there was still warping. This was dry wood.

These pieces have formulated acoustics (a la Computer Aided Didge Sound Design, created by Frank Geipel in Germany) which require precise internal dimensions to get it right, so it takes a few days to get the inside circumferences just right before glue-up. By the time I've got the dimensions carved just right the warping is in full swing. If it was just a traditional free-form didge there would be no warping problem as I could have it cut and glued back together in the same day. So I've really been looking for a way to deal with the warping and not have so much anxiety about the pieces remaining apart for too long. The warping not only makes for a bad joint and stressed wood if over-clamped but the gap in the joint (on the inside wall of the instrument) undoes all my tedious efforts to get the millimeter-precise circumferences for acoustics.

I know I'm making this into a long reply, but I've really had a lot of questions as the piece I'm working on has kind of been a disaster as far as the warped joint. Epoxy would fill in the gap on the inside of the joint, but it's too beautiful a log and I've put too much work into the acoustics to glue it up with the warping. First I tried steaming the pieces to get them back in line with each other, but this only made things worse as some parts of the joint started to curl outward instead of in. As it's dried, most of the outward curling has vanished. At this point I'm working the inside with grooves, epoxy filler and clamps to get the joint to sit flat again (which means I'm going to have to go and check all my circumferences again for acoustics purposes b/c they will surely have changed). The dowels are also going to help with the alignment.

Not sure about tinting the epoxy for the joint. Parts of the log are heartwood (dark reddish brown) and parts are sapwood (closer to white) so it might stand out badly in some places, but I"ll keep that in mind and maybe do some experimenting with it.

I'm really excited to hear you say that I can get an invisible joint with epoxy. I've been wanting to switch to epoxy for a long time. It would make glue up and inside finish coating a much easier process. Just to think, I could do glue up and finish coat the inside at the same time! It does worry me though because I still don't trust my epoxying skills. One of these days I'm going to find some consistency about it. I guess as you say the best thing is to test it out first.

On that last piece I mentioned (where the epoxy finish coat easily peels off the fine-sanded wood) I did as you said and applied all subsequent coats while the previous coat was green. From everything said, I think the main problems were 1) using solvent on the first coat, 2) epoxying while it was humid from rain and 3) sanding the wood too fine.

Again, thanks for the advice. And now back to the grind....

Excuse my ignorance but what is a Didge? I asume it is some type of musical instrument.
How about a photo of what you are making.
By what you described at first I thought you may be making a Didgerydo.
Cheers Mac

Yep, it's a didgeridoo. Here's a photo of the inside of the one I'm working on, after the bandsaw cut, marked up and ready to carve. And another photo of one I made last year, also of mtn mahogany.

The brand and type of epoxy is another issue that needs to be addressed.

The common retail epoxies are realy not a patch on the purpose made boating epoxies, such as West System, Boat Coat and System 3

these epoxies are designed specificaly to work with wood and penetrate very well..........they come as unfilled resin and you have to mix in whatever thinckener you chose to achieve the desired viscosity.....you get soo much more controll.

And these good epoxies will bond just as well clamped or not as long as the parts fit.....unless of course you over clamp....with epoxy use the absolute minimum clamping preasuer to bring the parts together.


Over sanding stuff is a very very common problem.

So may people oversand.......if you are using almost any film forming finish including laquer, there is no point sanding past P180...

Fine woodworking did some tests and an article on this and confirmed manufacturers recomendations 100%.

The only time you sand real fine is either after finishing or if doing rubbed finishes like oil and french polising.

Particularly so with hard dense woods..... the finishes simply will not stick.

cheers

Thanks for the additional input.

A friend who got out of the pebble rock flooring business gave me almost 15 gallons of epoxy a couple years ago. This is what I've been using. I never bothered to look at the brand, I just knew it was for flooring and 100% solids. After your post, Soundman, I went out and looked at the epoxy to see if I could find a brand name. It is called Seamless Final Coat Outdoor Sealer and it comes from Seamless Flooring Systems in Salt Lake City. Here is their link: Seamless Flooring Systems - Products I just called and talked to someone there to see if I could get more hardener since I still have 2 gallons of resin left (I recently spilled almost a gallon of hardener). He said they don't carry that formulation anymore and pointed out that shelf life could be a problem, especially if the epoxy has been exposed to winter weather--which it has.

From what you said, I gather that this is not the ideal epoxy for bonding wood.

I don't really have the money for the more expensive brands, so maybe someone could point out a brand that can compete with the others but for less money. AeroMarine Products (AeroMarine Products - Epoxy Resin, Urethane Foam, Silicone Mold Making Rubber) has 3 gallons for $132, which seems like a good deal if it is suitable for my application. I'd like to use it for both encapsulating and gluing the joint (using fillers?), but maybe I'd need to get a separate kind of epoxy specially formulated as an adhesive. West System's GFlex looks good--I don't really need a lot of epoxy for gluing joints, so I could go all out on the glue and get the expensive brand if it's going to make a big difference in bond strength. .

getting the right product for the job is always the way to go.

That flooring epoxy will be designed with stone and cement in mind and probaly a very thick film.

remember the west product is unfilled, when you add fillers you will extend its volume quite a bit........a small amount of properly preapred epoxy goes a long way.

remember too that with " expensive epoxy" you tend ( if you are wise) to be a bit more carefull with it rather tha slathering it about.

remember you can buy it in small quantities...I buy in 1 litre ( about a quart) packs ( 1 liter of part A and 200ml of part b...5:1 mix)
and I measure using 30ml suringes.......I often do mixes as small as 5ml.... unless you are playing with BIG things like boats....a 120ml mix goes a very long way.

If you think it is expensive.... compare it with retail epoxies like 24 hour araldite.....remember araldite is an old generation epoxy and is already got fillers in it.

Araldite is a great product for the retail consumer and the average tradesman....but it is a blunt instrument in comparison to West or any of the modern boating epoxies.

cheers

cheers