gaps in seams between veneer ironed on

I made some rather thick veneer (3/32 to 1/8 thick) and wanted to try out the iron-on method of coating the base and veneer pieces with PVA, letting the coats dry then ironing them on. It took quite a bit of time and heat (cotton setting and about 20 seconds in one place).
The pieces ironed on just fine, but the seam between each piece formed a gap. I tried to just hold one piece against the other, use a clamp to hold one piece against the other, veneer tape to hold one piece against the other. In each case, I had a slight (maybe 5 to 10 mils or so) gap in the seam. I finally used the old "many clamps" method and just glued the last veneer piece to the substrate without heat. That worked just fine. No gaps. I assume that the wood pieces lost moisture when heated and shrank leaving the gap.
Does the problem go away if the veneer is very thin, or is there another way to eliminate this gap problem.
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I haven't done it myself but I often see the veneering guys using super glue (cyanoacrylate (sp?)). Gluing the edges together first. Small amounts cyano with just a little pressure instantly cure to a very strong bond. Too much glue won't let the catalyst process heat up enough form the pressure to bond; so thin applications.
Just an idea.

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I usually use a plastic glue (unibond, but there are others) that dries hard as a rock. PVA will creep under continued stress and is no match for shrinking wood. I presume that thinner veneer would be easier to control.
MItch
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Eganders, The problem may be that the heat expands the veneer, then when it cools and contracts, it forms a gap. May be much less a problem with thin veneer that doesn't take nearly as much heat to bond the PVA. Not that I've ever done this. Kerry
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eganders wrote:

If you are using relatively narrow pieces of veneer to cover a wider surface you first make all the veneer pieces into one. You do that by joining mating edges then laying them out and fastening them together with veneer tape on the top surfaces (veneer tape is paper with mucilage on the back - some has holes, some doesn't, with holes is easier to remove). Strips of masking tape pulled tightly across the joint will pull them together while the veneer tape dries and keeps the top surfaces flush.
Once the tape dries, bend open the joints and coat with white glue. Bend back, wipe off squeeze out and let dry. Again, strips of masking tape across the joints can provide lateral pressure if needed. Once the glue dries remove any masking tape.
You now have a wide piece of veneer. If the veneer sheet is NOT flat, you need to make it so. I'll leave that for you to research (hint: Constatine's). If it IS flat, it is ready to glue to the substrate but I would NOT try to iron it on...especially one as thick as yours. Ironing (max heat) works fine for edging though.
The usual ways of gluing down the veneer sheet are vacuum press, veneer press, cauls, contact cement. If it is narrow enough you can just put a piece of 3/4 ply on top and edge clamp it. Here is another possible way for wider veneers. I have not tried it but intend to do so. I have used it in other applications.
1. Center sub-strate on a piece of 3/4 ply about 1-2"" wider on all sides than sub-strate 2. Apply glue to sub-strate 3. Position veneer on sub-strate 4. Cover veneer with protective paper such as brown wrapping paper 5. Put a piece of thick foam rubber same size as ply bottom on top of paper 6. Put another piece of 3/4" ply same size as bottom on top of paper 7. Edge clamp ply to ply all around.
The problem with wide veneers is getting pressure on the center of the veneer. I think the above would do so due to the foam rubber.
Regardless of how the veneer is attached, once it is dry you have to remove the veneer tape. You do so by moistening it, scraping it up then wetting & wiping off any residual mucilage. After sanding, you are ready to finish.
--

dadiOH
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eganders wrote:

I'm thinking your veneer is too thick for this method to work well. The amount of heat needed to activate the glue is expanding the veneer. When it cools off and contracts, something has to give and the seem opens up.
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This (Amazon.com product link shortened) />/ has multiple articles referencing this iron-on veneering method. A couple comments that might be relevant: a) some woods shrink under the heat, pre-shrink (heat) the veneers before applying. b) leave a small overlap when gluing down the middle of the "next" individual veneer piece, then trim and/or press down the edge of this piece that mates with a previous piece in a "second pass".
As other have mentioned, this would probably work better with thinner veneers... -- JeffB remove no.spam. to email
eganders wrote:

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