Finish under laminate?

[Jim Barstow]

I'm building something different from my usual greene and greene style furniture: a sewing table for my sister in law. The top is baltic birch 3/4" plywood with laminate. With solid wood tops, I'm always careful to finish both sides of a table top to prevent warpage. In this case, I'm not sure what to do. Finish both sides with water based polyurethane (endurovar) then apply the laminate or finish one side and assume the laminate seals the top well enough? I figure that the plywood will be much less prone to warpage but I'm building the table in dry California and it will be used in Tennessee where the humidity is about 300%.

[Bill White]

No need to finish under a laminate surface that I know of. I wouldn't even finish the reverse side when using BB ply.
Bill

[Jim Becker]

The "potentially right" answer really depends upon the specific workpiece and how it will be supported post construction. Just like when veneering, in some cases, folks will put a "backer" laminate on the non-visible side of a substrate to help keep things balanced relative to seasonal stress. Laminates potentially "move" just like wood does with change in temperature. Something like an unsupported cabinet door that will be laminated would absolutely benefit from a backer layer. But for a table top that will be constrained by how it's mounted to the table structure, that fastening will also help serve to keep it flat.

That said, directly under your laminate, you want a clean, dry, smooth surface so that the contact adhesive will do its thing correctly when you apply the laminate. Be sure you have a proper roller to help with air bubbles and having some dowels handy to help with placement before you "stick it down" starting at one end is also advised.

[Peter Kelly]

I figure that the plywood will be much less prone to warpage but I'm building the table in dry California and it will be used in Tennessee where the humidity is about 300%.

Laminate both sides with the same material. Plywood isn't any less prone to warping if left unbalanced.

[Jim Barstow]

Anyone know if the contact cement will work over water based urethane?

[Peter Quinn]

Just read the DAP weld wood can....remove all paint and varnish from substrate prior to applying contact cement. It might stick but it will stick to the finish, so the bond is only that strong. Test first. If the cement dissolves the WB lacquer that's bad. I've seen a few posts here recently suggesting you laminate both sides of table or counter top....which strikes me as curious because short of guys obsessing over router tables I've never heard of or seen a top with HPL on both faces. Doors yes, tops get screwed down to the structure. When you say "laminate" I'm understanding Formica or similar? In veneer work that has to remain flat balance is essential, glue bonds are rigid, wood moves. But contact cement is elastic to some extent, plastic is plastic, different thing.

[Jeff Duncan]

Jim and Peter have covered it pretty thoroughly. Follow their advice and you'll be OK. Try to experiment and re-invent the wheel and….well….who knows???

good luck,
JeffD

[Jim Becker]

Anyone know if the contact cement will work over water based urethane?

The "best" (relative term...) contact cements are solvent based and they are to be applied to clean, finish-free surfaces.

[Art Mann]

I agree with Jim. The only thing applying any finish to the plywood surface will do is compromise the adhesion of the laminate. How you should proceed depends entirely on the construction techniques. For example, I have built several plastic laminate top router tables and the first one warped like crazy. The reason is I only put laminate on the top surface of a double thick glue up of MDF. That happened a long time ago before I knew anything about dimensional stability. I have built several router tables since then with zero warpage (+/- 0.003") because I laminated both sides so that they responded the same to humidity changes. Having said that, I have also built several custom work surfaces such as you describe (plywood or MDF substrate) where I only laminated the top surface. I did finish the bottom surface with varnish. These surfaces have never exhibited any warp or bow. The difference is that the substructure under the table top was designed to hold the piece flat. I used screws rather than glue to fasten the tops on, which allows some movement between the top and the support structure. If you design your support structure correctly. you should not need to laminate both sides.

[Tom M King]

I've put it on one side of 3/4 Birch for decades. The lightweight, traveling outfeed table for our portable tablesaw only has two lengths of aluminum angle under it, and it's plenty flat enough. I've always glued it right to the plywood with solvent based contact cement.

[Paulh Tremblay]

I have a book on installing laminate on counter tops, and the book says nothing about applying laminate on both sides. I don't think I have ever seen a counter top laminated on both sides. Am I missing something?

[Jim Becker]

Most constructions using laminate don't need both sides done because of how they are supported/fastened down. There are likely some cases, however, where a backer would make for a more stable component if there is little support and the area is larger. Laminates and substrates can move at different rates...

[Tom M King]

I would need to see an example. I have a whole bunch of sink cutouts from the '70s that I still pull one out once in a while to make a specialized router table out of, and I've never seen one bowed. Maybe with something besides 3/4 Birch, but that's all I've ever used. Even the plywood will bow by itself if it's not stored flat, so it would have to be something left flat that bowed to prove that it matters. As long as the table is designed sturdy enough to keep the plywood flat, it doesn't need anything but the Formica glued to the plywood.

Back in the '70s, corners of the lips of the countertops were rounded with a belt sander, and the strip of Formica heated with a propane torch just enough to turn it around the bend. The edge was trimmed with a router, and then the top glued down, and it trimmed. Final finish of the joint was done with a block plane with a steeply angled bevel on the iron. Most carpenters kept two irons for their block planes. One for Formica.

[Peter Quinn]

I would need to see an example. I have a whole bunch of sink cutouts from the '70s that I still pull one out once in a while to make a specialized router table out of, and I've never seen one bowed. Maybe with something besides 3/4 Birch, but that's all I've ever used. Even the plywood will bow by itself if it's not stored flat, so it would have to be something left flat that bowed to prove that it matters. As long as the table is designed sturdy enough to keep the plywood flat, it doesn't need anything but the Formica glued to the plywood.

Back in the '70s, corners of the lips of the countertops were rounded with a belt sander, and the strip of Formica heated with a propane torch just enough to turn it around the bend. The edge was trimmed with a router, and then the top glued down, and it trimmed. Final finish of the joint was done with a block plane with a steeply angled bevel on the iron. Most carpenters kept two irons for their block planes. One for Formica.

My thinking is its contact cement, not glue. Its a permanent but flexible bond, so the wood can move and the plastic can move, but at different rates and not kill each other. Seems to be some confusion on that aspect, like the same rules that apply to wood veneer with a rigid glue line apply to HPL and contact cement. I like you explanation, drops from the 70's still flat. Might have been better plywood! They strongly suggest you DONT apply veneer with contact cement for the same reason they don't want you to apply HPL with glue. Veneer moves, wood moves, contact cement is to flexible to hold it to the substrate. But on HPL its like a decoupling membrane of sorts, lets each move separately without destroying the other. My kitchen counter is form the late 1950's, formica, still going strong, substrate is fir ply, absolutely no finish on the bottom side, still relatively flat, certainly not ruined.

[Art Mann]

I have seen inadequately supported homemade plywood or MDF laminated counter tops warp more times than I can count.