Laser char: are there any glues to avoid?

[bruce d]

I know laser char has been discussed before but I did not feel this point was nailed down: If you glue a joint where the char has been left on one or both surfaces, WHAT are the potential problems? For example, is there a glue to avoid? One that works well? 

I am building my first laser-cut kit (build log to follow) and many of the parts are so delicate that I only want to carry out the minimum sanding/scraping.

All comments appreciated.

 

Bruce

[BobG]

I think you will probably find a variety of opinions about gluing surfaces that have laser char, Bruce. I ran my own small test once where I glued some small pieces of wood together with and without laser char and they both ended up with strong bonds. 

 

Personally, I've had no problem with PVA or CA glues. I don't sand off the laser char unless it is necessary to obtain a better fit or if it I don't want it to be seen. Sometimes I'll sand it very lightly if it is extremely thick and heavy and/or dusty and loose just so I can get a cleaner surface for gluing. 

 

 

[bruce d]

29 minutes ago, BobG said:

I ran my own small test once where I glued some small pieces of wood together with and without laser char and they both ended up with strong bonds. 

Thanks Bob. I tried a couple of tests and was satisfied at first but was surprised the following day to see one of the CA joints had simply fallen apart. The other four CA joint test pieces were OK. There may have been sloppy practice by me and something affected that one joint but it seemed smart to ask the community.

I really do not want to attempt cleaning the char from two sides of the tiny one mm thick frames on the longboat I am working on. Of course, I also don't want the planks to spring off after the model is finished ...

 

 

[Gregory]

CA needs porous material to work well.  If the char is preventing penetration of the grain, it might be a problem.

[VTHokiEE]

7 minutes ago, Gregory said:

CA needs porous material to work well.

Really? I didn’t think CA needed a porous material as it is often used for metal bonds (I understand epoxy is another choice). Now PVA on the other hand I’ve always heard needed a porous surface.

[bruce d]

1 minute ago, Gregory said:

CA needs porous material to work well. 

I think that is behind the failure despite me doing a bit of scraping to allow penetration. Just needed more wood exposed, perhaps.

You may be able to tell, I find the tiny parts intimidating.

 

[bruce d]

On 9/28/2020 at 9:15 PM, VTHokiEE said:

Really? I didn’t think CA needed a porous material

Good point. I was told long ago that CA applied to wood needed penetration to form a solid mass within the fibre structure. Otherwise the few fibres forming the surface have broken their bonds with the fibres below. I simply believed the explanation, never questioned it. 

Now that the light is on the subject, it makes me wonder about the bond created with a wood-char-CA-char-wood club sandwich.

[Gregory]

21 minutes ago, VTHokiEE said:

Really? I didn’t think CA needed a porous material as it is often used for metal bonds (I understand epoxy is another choice). Now PVA on the other hand I’ve always heard needed a porous surface.

It may not always be the case, but I have never found CA to work well on, say, two varnished surfaces unless they were roughed up.

I have had success with laser char as long as the loose stuff was removed.

[BobG]

I use CA quite a bit and prefer it for planking especially. I can use just the smallest amount applied to the frames or bulkheads when planking. It allows me to move along quickly and without lots of complicated clamping. That said, I have found CA can sometimes be inconsistent and I've had to reapply a tiny amount in some instances. It doesn't always grab 100% of the time especially if too much is applied. Very small, thin amounts seem to work best and medium viscosity gives you a bit of working time.

[mtaylor]

I've found that a very light sanding with fine sand paper helps.  I don't try to remove all the "black" but just the surface rough bits.  And I don't use CA on wood if I can help it.

[wefalck]

Whatever glue you are using, just make sure that there is no loose carbonised wood dust left. The discolouring of the wood should not have an appreciable effect.

 

Personally, I stay away from CA unless it is a shape-locking bond and even then I prefer PVA. CA is easily sheared off, unless it really forms a structure that interlocks between the two pieces of wood.

[Canute]

Something else about laser cut wood parts. The laser cuts have a slight slope from the top to the bottom of a cut. Squaring these laser cut sides would help, but you don't need to remove all the blackening.

 

As the others have said, CA wants a tight fitting joint. A thick gel may work better in this case. And if you are gluing into end grain, you'll have issues, since the wood will absorb the CA faster than you can set the parts up.

 

Personally, I'd use PVA on the joints.