Hairline cracks appearing in rosewood veneer?!

[CroppyBoy1798]

Hi all.

Quick one.

So I'm producing dash panels lately for classic cars and using a santos rosewood veneer which is applied over a 12mm nine ply baltic birch plywood panel. All seems well until some time later, usually during the finishing process, small hairline splits appear in the veneer. These usually radiate from an edge or a cut out, they're not very long generally in the 5-10mm range. They do seem to appear if the panel is left in the sun before or after the finish is applied. I'd not mind so much but these panels are being finished to a very high degree with a 2K polyurethane over a polyester high build so I'm building them up to be cut back to mirror finish and even the small split opening in the veneer can ruin the finish and stick out like a sore thumb.

I'm applying the veneer using a cascamite powered resin adhesive, I've used this also for figured walnut, burl walnut, teak, cherry etc and have had no issues, it just seems to be the rosewood thats acting up.

Anyone else have any experience of this, a remedy or advice?

Cheers.

 

[profchris]

My guess is that you glued on the veneer in comparatively high humidity. As humidity drops the veneer contracts across the grain. The plywood doesn't (much, if at all). The resulting stresses are relieved by cracking at weaker spots.

Try checking humidity before glueing. Glue two strips of veneer together, one cross grain, the other long grain directions. 100mm long is plenty. Glue it to a block on a backing board and mark the tip position with a pencil. Then watch it move around during the day as humidity changes, and mark the extremes. The mark at the cross grain side is the dry side.

Once you can tell if it's humid or not, glue up only in drier conditions.

 

[D_W]

bake the veneer before applying it. Just lightly. This is a typical thing done for guitar tops where the same issue arises (except if the top shrinks, it's applied to bracing or other bits on the guitar so it can't move freely and then splits.

I don't know that that's done commercially, but an older luthier told me a while ago that he generally put all of his tops in the oven at a low temperature before gluing them, and never had an issue - the top will shrink a little over its life and never be a problem.

If you bake veneer dry (presuming the rosewood won't split, but it shouldn't) at a low temperature (not sure what 150? 175F?, it'll shrink pretty significantly and have good margin down the road (it'll be under compression a little after applied and gradually over it's life relax as size always loses the expansion and contraction war.

 

[Droogs]

You answered the question yourself- appear if left in the sun. The veneer is thin, the rapid drying out from the sun is causing the cells to shrink very rapidly and the cascamite, although dry has not cured to hold it in place. Really the panels should be kept indoors until they are properly finished. It takes your glue a week to fully cure. then of course you have to allow each coat of lacquer to cure properly too before cutting back and adding the next coat.

Are you using a reviver on the veneer before you use it to make it more flexible? If so give it a good heat - even a dry steam iron will do before laying/pressing it, that will help reduce shrinkiage once on.

If you do have to redo these it help to have the veneer for the panels a few mil oversize and trim to the correct dimensions once done and touch up the edge with lacquer to prevent moisture ingress. Rest of it looks good though. People do under estimate the power of sunshine to dry things out, even on a hazy day

 

[GuitardoctorW7]

Don't know if this will help but as for blemishes in the finish, I have recently discovered Gluboost products from Tonetech ™ Luthier Supplies
They do a CA accelerator that doesn't froth up. If I use water thin or medium viscosity CA it dries instantly to a hard clear finish that can be scraped/sanded and polished to a high degree. They also do a range of tints for for CA that can be used in lacquer and 2 pack. They go a long way but are quite expensive.
They are a game changer for me
Cheers
G