Veneer substrate for countertop

I'm building a walnut veneered "countertop" for a built-in office credenza, 11 feet long, 21" deep. I've found a source for 12 foot long veneer, so no problem there. The question is the substrate. For a plastic laminate countertop, I'd use 3/4" MDF (two lengths with a couple of joint connectors), and feel comfortable that I could maneuver the assembly from the shop to the office without it coming apart. I'm leery about doing it with veneer and have thought about overlapping 1/2" MDF sheets to make up a 1" top (there will be a solid edge-band). Other questions: Do I need to worry about warping and veneer the underside if it's screwed in well from below (think kitchen countertop), and what's the best veneer glue (no vacuum bag available)? Thanks!

If you’re applying standard commercial veneer, the key thing is that you have to apply pressure everywhere. The veneer is so floppy that if you apply a point pressure (like with a c-clamp), the veneer may pop up away from the substrate a few inches away. On small panels it is possible to apply pressure everywhere with cauls and clamps. On big panels, a vacuum press is the only way to succeed. It magically applies pressure everywhere.

Hi Phil,
I do similar commercial projects, mostly with Formica laminate. Just did a 10ft counter top in a 10 ft room, gave myself 1/8” at each end for the install! I join the MDF with buiscuits, if the added peice is not so heavy I’m confident it will be strong enough. If not I add a temporary cross brace under with screws to remove on safe arrival.

As for glueing veneer I have a dead flat assembly table about 25” high, braced up the ying yang. I flip the peice and pile sheets of MDF on top, lots of sheets, then any heavy stuff I have. Not the 15 psi of a vacuum bag but it does not squeeze out all the glue either. I use yellow glue or the waterproof kind if needed. Your project is longer than my assembly table, I would look for a large enough flat surface, a sandy beach after the tide leaves, a covid emptied mall floor, cast concrete bridge footing etc.

I have not done any veneering of consequence but I have used a fair amount of plastic laminate. So I ask this question as much to satisfy my own curiosity. Why not use contact cement to apply the veneer? No need for a vacuum bag, just roll the veneer from one end to the other in 12"-18" increments.

Thanks.

Brian, veneer tends to be very thin and delicate. Numerous kinds of contact adhesive of course, they don’t soak or flow into the wood grain for the bond, they are more likely to dry and fail with time (but not some). Harder to apply evenly so may cause an irregular surface. The open time is very short, with instant grab. My most used contact adhesive for commercial melamine work is a high temp adhesive in a large ‘propane’ tank. Even with the expensive oscillating spray head the coverage is irregular. Both surfaces are sprayed, it dries & bonds for a minute to each surface then they are joined. The irregularity does not show through commercial melamine but veneer would have no chance.

I would not be surprised with time if discolouration appeared in veneer from the contact adhesive.

Consider subbing out the job. There are plenty of millwork shops that do this stuff every day and have the equipment to do it properly.

If you are intent on doing it yourself paper or phenolic backed veneer is probably your best option. You can use contact cement on .020" paper backing or phenolic, though that wouldn't be my first choice. You can use epoxy with cauls and clamps or weights for a glueline that is less demanding of absolutely uniform pressure and will give you enough time to roll it out and get the pressure on. If you use epoxy abrade the mating surfaces with 80#. There is a pva glue known as FSV (for Flexible Sheet Veneer) that can be used with a pinch roller or pressed down by hand for paperback. Don't use contact cement on raw veneer, it will come back to bite you.

If you do use raw veneer you will have to seam the pieces together using some kind of long straightedge jig (unless you sourced some wide rotary cut material). Your best bet may be a tracksaw with carefully aligned track sections. I have used a router for 12' seams but obviously you need a means of making the straightedge to guide it. The leaves can be held together with veneer tape.

The substrate can be built up of 1/2" layers with staggered joints or a single layer with a double row of biscuits or similar. Glue the butt joint well and transport with temporary longitudinal cleats on the underside. Best practice is to treat both sides the same, using a species with similar movement potential or reject material from your flitch on the underside. With paperback you can get by with a thin phenolic backer sheet like Gatorply.

If you want to do veneer work on a regular basis at this scale you will want a fliptop vacuum press at the minimum. For raw veneer you will want a guillotine or veneer jointer and a stitcher. The gear to do long assemblies like this is expensive. If it's an oddball job, sub it out. Veneer is a completely different process from p-lam.

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Originally Posted by William Fretwell

Brian, veneer tends to be very thin and delicate. Numerous kinds of contact adhesive of course, they don’t soak or flow into the wood grain for the bond, they are more likely to dry and fail with time (but not some). Harder to apply evenly so may cause an irregular surface. The open time is very short, with instant grab. My most used contact adhesive for commercial melamine work is a high temp adhesive in a large ‘propane’ tank. Even with the expensive oscillating spray head the coverage is irregular. Both surfaces are sprayed, it dries & bonds for a minute to each surface then they are joined. The irregularity does not show through commercial melamine but veneer would have no chance.

I would not be surprised with time if discolouration appeared in veneer from the contact adhesive.

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Thanks William. Appreciate the details in your reply. Learned something today so I can sleep tonight.