disassembling a hardened PVA glue-up.

[MJP]

Disaster!
I glued up a small assembly last night and this morning I found that it had got out of square and needs to be disassembled.
I've read the thread back in July 2012 on this subject but I wonder if there's any new ideas on how to separate this glue-up?
The glue I've used is Evo-Stik Resin W.
My first inclination is to try a hot air gun on the joints - the piece is only about a foot square so it's not a big job - and failing that, pouring boiling water over it.
Any better ideas?
Martin.

 

[MJP]

Well, I can answer my own question.
The glued areas were about 2cm wide and a foot long (yes I use the "Imperial Metric" system!) and a hot air gun applied for about five minutes followed by careful application of a knife down the contact line lead to success.
The glue lines bubbled and turned brown and the wood (walnut and pine) was just on the edge of singeing.
The three pieces of wood separated cleanly, without damage.
So dry heat will separate glue-ups for small jobs.
Now to clean up the mess and start again!
Martin.

 

[profchris]

You probably didn't need so much heat!

Luthiers know that PVA and aliphatic resin glue such as Titebond softens at the kind of temperatures you find in a car on a hot day.

If you need to do this again, try less heat but for longer - it takes quite a time for heat to penetrate 2cm of wood. Once the heat has gone all the way through, your knife should do the trick equally well.

 

[MJP]

"If you need to do this again, try less heat but for longer - it takes quite a time for heat to penetrate 2cm of wood. "

Thanks Chris.
Yes, I thought that this might be the case. But I was fuming at my error and in some haste to undo it, so heat gun it was.

I couldn't think, in the heat of the moment (!!) how to warm the joints more gradually - I couldn't put the whole thing in the oven for example because there are other glued joints which I wanted to preserve.

As for "the kind of temperatures you find in a car on a hot day" - I'm in South Wales, we only get a hot day every leap year, can't wait that long.

Thinking about it now, I wonder if wrapping the glue-up with soil warming cable might be a good idea? That would be an easy way to apply gentle heat to a restricted area over a longer period.

What method do the luthiers use?

Thanks again Chris,
Martin.

 

[profchris]

Luthiers tend to use a domestic iron, or hair drier/heat gun, or heat lamp, or heat blanket for things like the bridge.

The challenge is the neck to body joint, as the neck block is maybe 50 X 100 mm and they don't want to unglue it from the dies, back and top. Here they drill into the joint and insert heat, via steam or a heat stick.

If I'm building and need to disassemble I often use a heat gun. Once the finish is on, something gentler!

 

[custard]

Titbond sell a PVA variant called "Extend", it's marketed on having a longer open time.

But in truth it has a very small amount of additional open time, where as it has a LOT more resistance to heat! So much so that in the trade it's the first choice for gluing up fire surrounds, mantlepieces, radiator covers and other jobs subject to a lot of heat.

Disassembling a job glued up with TItebond Extend is a bit more of a challenge!

 

[MJP]

Chris and Custard - thanks both.
Interesting to see how others do it..I'll experiment a bit with unglueing joints at some time when I come across suitable warming equipment, though I don't think SWMBO will lend me the iron!

Seeing that I'm likely to make a lot more of these misalignment errors shortly Custard, I'll stay well away from Extend!

 

[MJP]

Would you believe - I made the same mistake again!

This time I placed the glue line under the grill of our gas oven, with the heat turned down as low as it could go.

After ten minutes, turning the joint a couple of times, the glue separated easily without any signs of overheating, though the kitchen had a lovely woody aroma.

It's amazing how little glue is actually on the glued surfaces, a brief scrape after it's cooled down is all that's needed to give a clean fresh surface for re-glueing...correctly this time, I hope!

"Never in the history of human glueing have so many mistakes been made over such a small job"

Martin.