Help with burr veneer please

[flh801978]

I have tried to veneer a panel 400mm x 300mm with a pair of book matched burr walnut veneers
Used a vacuum bag and pva glue
After removing the panel from the vacuum after about 60 minutes it was obvious that the vacuum has pulled the pva through the veneer and stuck it to a board of cardboard that I'd used as a caul
However there's a large ish area where the veneer has ruckled up and is a washboard surface about 250mm long but only 40mm wide
Everywhere else the veneer is nice and flat

I have 6 more sheets to try
Any ideas on how to avoid this the sheets are fairly flat to start with
What glue is best for this
Or is there anything i can do to repair the ruckle?

Ian

 

[Woodmonkey]

I recently used some burr walnut veneer on a sideboard. I used cascsmite, which I tinted with brown umber pigment. I used a sheet of plastic on top to stop it sticking, a plastic bag would do for something smaller. Alternatively, use a piece of mfc for your caul as the glue won't stick to it. Cardboard is not thick enough imo.

 

[yetloh]

Burrs are often difficult but the secret is to flatten the veneers before you start. I have used the method described in Bill Lincoln's book on veneering with great success. This involves soaking the veneers in a rather weird and wonderful potion and then pressing them before use. The big advantage so far as I was concerned was that, besides flattening them, the process changed the character of the veneers to something much more like a stiff leather. This made them much easier to handle and a great deal less prone to cracking and breaking which can be a major problem with burrs, particularly if complex shaping is required. It also seemed to make the veneers virtually immune to shrinkage which is another problem I experienced when using more simple water based methods of flattening. No doubt others will come up with their solutions but I could look out the formula if required.

Jim

 

[flh801978]

Wood monkey
If I,m using a vacuum bag do I need cauls that are thick? I imagined that thinner ones would help exert pressure evenly more?
What do people think about using gorilla glue ?
Jim
I,d be interested in the formulation of the magic liquor

Ian

 

[Droogs]

hi ian,
Unfortunately, there's not a lot you can do with the piece already done. Using PVA and other modern glues limits what you can do. One suggestion is if the glue layer is very thin then you can try heating it with an iron and just iron the veneer down. You may just get a way with this, but you must put small pin pricks in the part to be ironed. this will let some of the gases from the glue to vent. you may get a wrinkle at one edge and have to sort that out. this all depends on the properties of the particular pva you have used. You are trying to recreate an iron on veneer strip basically. it may not work.
The best way to avoid this is to really prepare the veneer before you start. you can't use it if its fairly flat it needs to be flat. the best way is to either soak in some water or bought in veneer softener until it is damp and then to sandwich in several layers of the paper the chippy wraps its chips in. then preferable place in a heated veneer press and put the veneer under the highest pressure you can manage and leave for a week. When it comes out it will be flat. You can then prep the bookmatch edges for glue up.
If you are going to use pva and the bag again, then tape up the match edges before you put on the thinnest smear of pva you can to the ground. Lay the veneer and place some cling film over it a sheet of paper and then put on the platten you want to use. When using vacuum and modern glues, i don't use a truely flat platten, i use one with the slightest of convex curved face - not really even visible to the eye - this does help prevent things moving i find.then seal and away you go.

I would however suggest you get some hide glue and try using that, at least that is by nature reversable with the application of heat and patience.

good luck

edit
if you don't have a heated press then just use extra sheets of paper and change them every couple of days until they are no longer damp

 

[custard]

There's a few different things going on there.

As the vac bag pulls up tight it can drag across the veneer, rucking it up, especially as the PVA will have come through right at the very start of the process and begun grabbing at the cardboard. Try smoothing down the vac bag as the air is being expelled, maybe even pausing the machine to smooth out any bag folds or ripples. As Woodmonkey said, use a plastic sheet, I use really heavy stuff like this, I find the thickness helps to produce more even pressure and a better bond.

http://www.ebay.co.uk/itm/2m-x-4M-Thick ... xyRNJShLPB

If PVA comes through the veneer it can mess up your finishing, that's one reason why some (not all but some) workshops make it a rule to never apply oil finishes to veneered work. A UF glue (or room temperature hide glue) is usually a better choice for burrs, and if you're using the glue to fill burr voids you'll have to tint it. Although you can follow a two step process and fill the voids separately before laying the veneer. You can also completely cover the face side with wide bands of sellotape, hold the veneer up to a window, and patch any voids and pinholes from the glue side with tinted epoxy.

Also, you say your veneer is flat, but how flat is flat? If I'm seeing ripples of more than about 4mm I usually flatten the veneer before laying. It's a faff but not as much of a faff as stripping the veneer off when it's all gone Pete Tong!

One last thing, you will almost certainly now have bits of dried PVA on the inside of your vac bag. Try and clean it off or it can cause problems on the next press. I know some really excellent veneer craftsmen who have absolutely filthy vac bags, somehow they manage but I don't know how because unless it's all scrupulously clean I'll often hit problems.

Good luck!

 

[Woodmonkey]

I flattened mine with a mixture of water, glycerin and meths, some folk also put some PVA in the mixture, I think this helps it stay flat if you're not going to use it straight away.
The vacuum pressing book I've got he was using 18mm mfc top and bottom of the workpiece, with grooves cut in the bottom piece to allow the vacuum all around the piece.

 

[custard]

What do people think about using gorilla glue ?

Not a lot.

There's only four adhesives you'll ever need for press veneering, epoxy (for when adding moisture is a no no), UF (Cascamite is a UF, but if you're doing any kind of volume then liquid UF with powder hardeners, like Bordens, are better as they open up all sorts of options, like thickeners to reduce bleed through, and different speed hardeners), room temperature hide glue (because it's repositionable), and PVA (because you can get the job out of the vac bag after only 30 or 40 minutes).

 

[custard]

The vacuum pressing book I've got.

What book is that Woodmonkey?

I've met a few American woodworkers and they seem to go for more elaborate platens with vac bags, I don't see that so much here. In fact I don't even groove my platens, I just rely on breather strip.

I think the whole vac bag thing is so new (in woodworking terms) that there's no settled consensus on the best approach. There are some dramatically different methods being used by different people. Probably because some methods give advantages with different equipment and for different applications, and maybe also because some people learned veneering long before vac bags came along and so cling to either hydraulic press techniques or hammer veneering techniques.

How did you get on with your heated pad experiments?

 

[Beau]

Don't know what you can do to salvage that one but when I do veneer I put a platen over the veneer after having similar problems to yourself.

So this is the build up I use for flat veneering

Substrate
Veneer
Release film
Platen
Breather fabric

 

[Mr T]

I would flatten a veneer if the distortion was greater than about 4mm. Here is the flattening method I use, cut and pasted from my veneering and laminating handout, with acknowledgements to Richard Jones.

Recipe
2 Parts PVA glue
3 Parts water
1 part glycerine
1 part methylated spirits
Method
This method can be used to flatten any number of veneer leaves in a stack.
Essentially the veneer leaves are sandwiched between paper with a layer
of porous netting preventing the veneer from sticking to the paper.
• Liberally brush the mixture onto both sides of each veneer leaf and
allow any surplus to soak in or run off. Leave for about ten
minutes.
• Lay a double sheet of newspaper or lining paper on an MDF board,
cover this with a layer of fibre glass matting or fine mesh netting.
• Lay a veneer leaf.
• Repeat a layer of matting or net then more newspaper.
• Repeat this for each leaf of veneer to be flattened finally complete
with a second MDF board.
• For badly buckled veneers exert pressure on the stack lightly .
Increasing the pressure progressively. In this way you avoid cracking.
For less badly buckled veneers use the veneer press or tighten the
clamps immediately .
• For the first 24 hrs renew the paper about three times. In the second
24hrs the mesh can be discarded and the paper renewed a couple
of times. All the time maintaining pressure. Put the dampmpaper on a radiator to
dry out for reuse.
• At the end of the second day the veneers should be flat and dry
enough to use. If you use them too early the moisture content will be
too high and the veneer may move after laying.

One problem I have had with this mixture is minute black spots developing. I thought at first it may be steel contamination but now I think it may be some kind of fungal stain. It's easily removed with oxalic acid. Perhaps Richard (Sibn Duhb or whatever he's called) will be along with more detail.

I don't think the corrugation you are getting is due to the veneer not being flat to start with, I think this developed during pressing because the card board is not hard enough. I would recommend using thick cauls top and bottom (18mm mdf perhaps). To avoid the caul sticking to the work cover it with strips of parcel tape.

Chris

 

[n0legs]

Could you guys who have mentioned "breather strips" and "release film" provide some links please?
Only so I know I'm buying the right stuff.
Thanks in advance.

 

[Beau]

Could you guys who have mentioned "breather strips" and "release film" provide some links please?
Only so I know I'm buying the right stuff.
Thanks in advance.

I just use a poly bag for the release film but bought the breather fabric from a place that does parts for glider repairs and have lost their details but hopefully someone else will have come across them. Really nice and friendly service and good prices.

 

[Mr T]

Could you guys who have mentioned "breather strips" and "release film" provide some links please?
Only so I know I'm buying the right stuff.
Thanks in advance.

I just use a poly bag for the release film but bought the breather fabric from a place that does parts for glider repairs and have lost their details but hopefully someone else will have come across them. Really nice and friendly service and good prices.

I use parcel tape as a release film. The breather strips are to distribute the vacuum pressure and stop the outlet tube sucking up to the wall. I just use and old piece of woollen blanket. Mine is a more low tech. workshop than Beau's!

Xhris

 

[custard]

This is the breather fabric I use,

http://www.airpress.co.uk/269-breather- ... -wide.html

I know a guy who uses some sort of upholstery stuffing material together with strips of MDF instead (MDF is very porous) and he seems to get pretty good results.

 

[yetloh]

The recipe quoted by Chris T is similar to that suggested by Bill Lincoln but the latter's includes some Cascamite instead of PVA and also has some white flour. I suspect one of the most important partsof either recipe is the glycerin which will soften them and also make them less absorbent of water, both of which are valuable alterations to veneer properties; the first makes them less brittle and generally easier to handle and the second reduces the risk of joints opening or veneer splitting as the veneer dries. I agree that PVA should be avoided like the plague. Apart from bleeding through, if you doget any bubbles ar areas of poor adhesion you can't solve it by injecting more PVA because it doesn't form a good bond with dried PVA; I use UF glue. While a heated press is the ideal I have found that my Airpress with thick ply or MDF cauls and breather fabric is perfectly satisfactory as long as the burr veneers is treated first.

Jim

 

[n0legs]

Thank you gentlemen =D>

 

[Woodmonkey]

The vacuum pressing book I've got.

What book is that Woodmonkey?

I've met a few American woodworkers and they seem to go for more elaborate platens with vac bags, I don't see that so much here. In fact I don't even groove my platens, I just rely on breather strip.

I think the whole vac bag thing is so new (in woodworking terms) that there's no settled consensus on the best approach. There are some dramatically different methods being used by different people. Probably because some methods give advantages with different equipment and for different applications, and maybe also because some people learned veneering long before vac bags came along and so cling to either hydraulic press techniques or hammer veneering techniques.

How did you get on with your heated pad experiments?

Can't remember what it's called, I'll have a look next time in the workshop. It is an American book. I have tried his way, and also just using breather fabric and both ways work fine.
I haven't got round to any experimentation yet, although I did throw an electric blanket over my sideboard top when pressing as it was pretty cold in the workshop and was worried about v the glue going off, it did feel pretty toasty under there after a couple of hours!

 

[custard]

I did throw an electric blanket over my sideboard top when pressing as it was pretty cold in the workshop and was worried about v the glue going off, it did feel pretty toasty under there after a couple of hours!

That's got to be the way forward, with vac bagged UF in particular.

I can't quite get my head around the price difference between this,

http://www.airpress.co.uk/315-heater-ma ... -soon.html

and this,

http://www.johnlewis.com/dreamland-1633 ... n/p2999425

 

[Sgian Dubh]

I can't quite get my head around the price difference between ...

I suspect the price difference may be caused by the temperature the mats or pads from AirPress are capable of reaching safely compared to the electric blankets from John Lewis. It wouldn't surprise me to find the AirPress items can heat up rapidly to ~60ºC, this being the sort of temperature used in hot presses. Even veneering with urea formaldehyde at this temperature means the veneered panel can be removed from the press after less than five minutes. Slainte.