Which substrate for veneering?

[Woodmonkey]

I have obtained some rather lovely consecutive leaves of burr walnut which i will be using to make a sideboard-come-bar type thing. I've only done quite basic bits of veneeing before just using mdf, but I've seen mentioned on here (by Custard among others) of using a sandwich of ply/ mdf. What's the advantage of this, and what's the procedure? I'll be attempting quarter matching for the first time so any other veneering tips gratefully received...

 

[custard]

Moisture resistant MDF is a beautiful substrate for veneering, but it's not very strong, so if you're sinking Dominos or loose tenons into MDF the joint may not be strong enough to withstand all that many house moves. It's also not the best material for fastenings like screws, yes there are specialist MDF screws, but they make a bad material tolerable rather than turn MDF into something really robust.

Good quality birch ply isn't a great substrate for veneering, it's often not all that flat, nor is it particularly consistent (you can find variation of half a mill in the thickness within an individual sheet in even good quality birch ply), plus you can get problems with defects in the ply telegraphing through the veneer. But it's plenty strong enough to guarantee really strong joints, and it takes fastenings almost as well as solid timber.

Consequently many makers combine the best qualities of these two different materials. You use ply for example as the core of an apron rail which will be attached to a solid timber leg, but then skin it with 3mm moisture resistant MDF on both sides, apply a matching solid timber lipping to the underside, flush off the lipping, and then veneer over both faces. This will give you rock solid joinery and a high quality veneered surface.

The fly in the ointment is that 3mm moisture resistant MDF is a special order line, there used to be loads in circulation but for the past year no one has met the minimum run requirement (apparently it's several truck loads), so there's now none available. This gives you a choice, 3mm standard MDF or 6mm MR MDF. For something like a sideboard I'd go with 3mm standard MDF and just hope a cat doesn't knock over a flower vase full of water!

I'll post a photo tomorrow when I'm back in the workshop, showing cross sections through test components built up in this way.

 

[marcros]

beware of light coloured veneers over MRMDF- I have had a slight green tinge showing through in the past. It may have just been my eyes, but I am sure that I could notice it.

 

[custard]

beware of light coloured veneers over MRMDF- I have had a slight green tinge showing through in the past. It may have just been my eyes, but I am sure that I could notice it.

Interesting point. It's possible on very thin commercial veneers, but it could also be a reaction between the timber, Sycamore in particular, and certain UF adhesives. The last few years have seen a big increase in the use of Rippled Sycamore and quite a few makers reported "green tinge" effects, the solution seems to be adding thickening agents and accelerants to the adhesives.

 

[Woodmonkey]

Shouldn't think it will show through this craziness...

Would you still need to put a balancing veneer on the back with all those layers?

 

[marcros]

beware of light coloured veneers over MRMDF- I have had a slight green tinge showing through in the past. It may have just been my eyes, but I am sure that I could notice it.

Interesting point. It's possible on very thin commercial veneers, but it could also be a reaction between the timber, Sycamore in particular, and certain UF adhesives. The last few years have seen a big increase in the use of Rippled Sycamore and quite a few makers reported "green tinge" effects, the solution seems to be adding thickening agents and accelerants to the adhesives.

it was (commercial) rippled sycamore, so at least i was in fashion! But it was with hide glue I think- either hot or the titebond liquid stuff, I cant remember which. I suspect it was me. It started at 0.6mm, and had a very light sanding, but even that would have made it slightly thinner still, so was probably 0.4mm when done...

 

[Peter Sefton]

MDF is my favourite for veneering onto, Birch ply has lots of issues as pointed out by Custard and it goes into twist.

We have just completed a sideboard that will be at CCD this weekend, and it was featured in Wood Working Crafts magazine issue 17.

This was in ripple sycamore which as mentioned before is a pipper for green tinge or even brown from standard MDF when next to clean white sycamore lippings. For the sideboard we used an MDF core with 1.5mm birch ply either side to stop lipping telegraphing.

I have moved away from using MR MDF for the moment (unless going into bathrooms or kitchens) We have had really deep green which has caused us a few telegraphing issues. My thought for furniture in standard conditions is finished with a good water resistant polish and veneered with UF the MDF is isolated from the moisture. We always leave our spare UF in the mixing pot to see how it has cured, once gone hard there is no way water will penetrate it. I feel this gives the MDF a good barrier.

PS, always balance.

Cheers Peter

 

[custard]

Here's an example of how a furniture component might be built up for veneered work. Obviously different components will need different arrangements, but this example illustrates many of the general principles.

This is a side apron for a desk. The overall thickness needed to come in at 50.0mm to exactly match the leg thickness, so the ply core was built up to about 42mm then hand planed and taken through a drum sander to get it absolutely flat and to a finished thickness of 41mm. It's then skinned on both sides with 3mm MR MDF.

Next a very thick bottom lipping is applied, this needed to be thick to accommodate a curve in the bottom of the apron. The lipping is flushed off so it's absolutely flush with the MDF.

Next saw cut veneers are applied (if you look closely in the photo you can see the veneer runs over the bottom lipping) and then flushed off, taking particular care at the top as another lipping is then applied (the top lipping) and then that is flushed off with the side veneers. This illustrates one of the advantages of making your own saw cut veneers, the lippings and the veneer all taken from the same boards, so you're guaranteed a perfect colour and grain match.

This is an off-cut from the finished component blank which is being used to work out the final arrangement for the joinery that will attach the apron to the leg, I seem to remember I squeezed in one more loose tenon and a tiny 4mm dowel to make the final joint rock solid and prevent any risk of gaps opening up in future years.

 

[Woodmonkey]

Peter, excuse my ignorance but what do you mean by "telegraphing"?
Interesting how you two have the core/ outer reversed...

 

[Peter Sefton]

Peter, excuse my ignorance but what do you mean by "telegraphing"?
Interesting how you two have the core/ outer reversed...

Telegraphing is were the substrate shows through the finish veneer, often seen when a different coloured lipping is used to the MDF/Ply core.

Sorry, didn't mean to contradict Custard, but one solution doesn't fit all circumstances. Custards illustration is a load bearing rail which needs good jointing strength, an MDF core would most likely fail in his job. Custard also has a drum sander which opens up different making techniques.

The job I was referring to has laminated curved doors, the core door was 5mm flexiply 3 layers with the 1.5mm Birch on either side, we used the MDF core for the rest of the project.

The maker has to assess each job on it's merits and which tools, techniques and materials he has access to.

Cheers Peter

 

[custard]

Interesting how you two have the core/ outer reversed...

Peter's explanation is absolutely right, it's about fitness for purpose.

My example is an apron that supports a leg which has no support from stretchers, so every time someone bangs a hoover into the base of that leg it will act as a lever and try to pull apart the joinery at the apron, consequently it needed something beefy and bombproof.

 

[Giff]

Slightly off topic, but connected. What glue / adhesive is best if using a technique like this in a vacuum bag ? G

 

[Woodmonkey]

I think i will be using uf glue (eg. Cascamite)

 

[Giff]

Will that cure in a vacuum bag ok ?

 

[Woodmonkey]

Yep, sure will, and it has a longer open time than say pva, so you have a bit more time to faff around getting everything lined up in the bag.

 

[Giff]

Great thanks. I haven't used Cascamite for a while.....since it changed it's name from / to the dubious Extradite ! Thanks

 

[Peter Sefton]

This is what I would recommend for home woodworkers, we use this in our school but other more commercial UF for a quicker cure time.

http://www.polyvine.com/index.php?optio ... 84&lang=en

Cheers Peter