Troubleshooting Cupping in a Bar Top

A solid Ipe bar top starts to show cupping. Furniture-makers chime in on wood movement and the nature of the problem. January 13, 2006

I need to find a way to solve this problem. I have designed and built a custom bar for a client with the top made from IPE wood. I am getting some slight convex cupping of some of the individual boards where they were initially a flat, smooth surface. The link below shows some photos of the problem and construction details. Needless to say I did a lot of research before, during and since starting this project. The design alone was about 1-1/2 yrs. My inclination is to let it settle in its new home and then sand and refinish. Would the problem recur? How long would it take to completely settle? Is there another solution? I don’t have the option to remake it. Has anyone experienced and fixed a similar problem?
Related Web Page: Photos of Problem and Building Specifics

Forum Responses
(Furniture Making Forum)
From contributor A:
Did you check the MC with a meter? I have not used Ipe before, but I know locally it is sold as a good deck wood, which would be an indicator to me that is doesn't need to be KD, if it is going to lay out in the rain. Even if you air dried it for two years, it probably would not be below 12% MC. Then when it goes into an AC environment, it will likely end up 6 -8 %, but that would come somewhat later. The convexity that you are seeing now is not from the surface drying that fast in your mild climate. I suspect it is from the water in the Titebond 3 slowly absorbing into this dense wood, swelling the underside. Do I see a little opening of the gluelines on the surface? Is this worse than when you finished?

Your biggest mistake is treating this solid wood top as if it was a veneer. The reason to cut veneer thin and glue it to a stable substrate is to stop it from moving with shifts in relative humidity. If they are not thin, less than 1/16", the wood has too much structural integrity for the glue to be able to hold it. When it is this thick, if it doesn't break your glue line, it will tear chips off of the face of the wood, and fail anyway. You can not stop solid wood from moving. You need to learn to plan for it to move, or learn to veneer with the thin stuff. Both are good options, but not both at the same time.

Your project is just beginning to self destruct. My best advice to you now is to look at that last year of planning and research with a bit of skepticism, and see what you can learn from your mistakes now. If you had not glued it to the p/w substrate, the outer band would have been a mistake also for the solid wood option, but it would have been fine for real veneer over a stable substrate. If you believe that only solid wood is the real thing, you should have milled either T-slots or dovetail ways in the bottom and waxed the parts so they could move without warping or binding. Those parts could be connected to the carcass, while the top moves freely.

When I am teaching, I always tell my students that you can learn more from your mistakes than from accidentally getting things right without understanding why. Of course it is unfortunate to have to remake expensive parts. I hope my assessment is wrong, but I suspect it is going to get worse.

From the original questioner:

I did not use a meter. Using the same wood successfully in other cabinet applications over the past two years lulled me into a sense of security about its stability. It is extremely dense, hard, heavy and oily and that was more of a concern, though I understand the seriousness of moisture.

As for your second question, no - as of today there is no cracking at any seam at all. What you are detecting is something that you can feel as you rub your hand across the surface. The finish seems to be compressing and rising very slightly at the joints. I appreciate your candor in assessing a worst case scenario that may be imminent, as well as the very good advice that you offer.

As I began to notice the negative changes I began to call some of the Ipe yards and product manufacturers that are located around the country to ask for opinions and advice. The consensus was to leave it alone, wait and it would probably settle. I came to the knowledge bases and forums in case their optimism was wrong so that I might possibly come up with a positive pro-active solution. Worst case scenario is also good.

Since you do not address removing the finish on top to let it settle, as one of the Ipe guys suggested, what about removing as much ply from beneath in areas that will not show? I stress again that what I am looking for are solutions and alternatives to failure if possible.

From contributor B:
I agree with contributor A on his analysis, and also the worst-case scenario. Several of the basics of wood have been overlooked, and this will become a rustic bar top in short order. This would have to be built with kiln dried lumber, and preferably something other than the notoriously hard to glue Ipe. There are two ways to make it: 1. solid wood, with fasteners that would allow for seasonal movement, or 2. as a thin shop veneer - 1/8" or so on both sides of a stable core (MDF, ply or similar) and then edge banded. Salvaging wood from a deck build is not a good point of departure for the amount of work evident in this bar.

From contributor A:
Here is a little quick thing that you can do to help you understand what is going on down under. If you have a thin veneer sample of this wood, or any other wood for that matter, cut a sample of it to 5 or 10" exactly. Apply the same amount of glue as you did under your top, but don't stick it to anything. Just observe what happens to it for a day. If it is handy you can measure it every hour or so. If it is too curled away from the glue, don't break it just to measure it. Maybe you can measure it when it goes back the other way. I know that this may sound stupid, and like I am giving you extra homework, but it is quick, and will help you understand this wood.

I suspect that you used a lot of glue under there thinking it would need a lot to compensate for any imperfections. I know a fellow who made a mistake like yours. He was sure he had some faulty glue, and after talking to the glue hotline, he took his vast knowledge of glue, and added a powder glue to his Titebond to improve it, then made another top using the same method, only to have it fail.

From the original questioner:
I agree contributors A and B that in hindsight I have made some errors that are major. I can say that in more than 25 years of woodworking this is the first major and potentially fatal mistake of large proportion. I have made plenty of mistakes over the years that I have learned from and have been able to find a solution to salvage the situation. I will never again make the same mistakes that I have here.To improve the outcome, what if I took the top off, and removed ply to expose the IPE on the underside to allow moisture to escape?

From contributor B:
I don't think removing the ply from under the top is going to help, because the edgeband (cross grain construction) will then restrict movement and cause at least minor self-destruct. Considering the difficultly in removing the ply, I would just go make a new one. Even at give away hourly rates, it would be cheaper. I have learned that it is better to go the whole mile in a complete redo than to try salvage something and have it fail again.

Remember the two rules of woodworking:
1. Wood will always move in relation to any humidity/moisture changes, period.
2. Woodworkers will all disagree on how wood behaves in certain circumstances, when subjected to moisture and humidity changes.

From contributor C:
Ipe can be an unpredictable species to work with. An important thing I have learned over the years is balancing panels. Whatever I do to the finish side, I do to the back. The panel must be symmetrical. For Ipe I would use epoxy, such as West System. As for finishing, if I planned on rubbing out multiple coats on the face, I would do a similar procedure to the back. When rubbing finishes, especially with a buffer, you induce surface tension and other stresses in the finish film. These stresses usually pull the cup up, or hold water. I have seen the opposite as well, probably due to unbalanced moisture. I would just let it sit for some time, see what it does and go from there.

From contributor D:
You’ve got nothing to lose by letting it sit. It may just need sanding and finishing, and meanwhile you can work and save up so you can afford the repair job if you need it.

From the original questioner:
You are all correct in your individual observations. Luckily, the clients love it so much that they feel that the minor flaws are not bad. I have spoken to them about all of the possibilities and assured them that if necessary I will rebuild at my own expense.

From contributor E:
I agree that the problem is treating solid wood as if it were veneer. There are different opinions on maximum thickness of shop-made veneer. I have been comfortable with up to 3/32", although Ipe, being dense and prone to checking, may do better at less thickness.

If the piece has been in service for a while (more than a year) without the problem getting worse in a stabilized humidity environment, and you can count on the humidity remaining stable forever, you might get away with sanding and refinishing. That is a lot to ask. One possible solution might be to run the top through a wide belt sander to reduce the top veneer to an appropriate thickness and remove the bottom veneer, and immediately replace the bottom veneer with an equal thickness of Ipe as is left on the top. You would have to redo the edgebanding or add a separate apron to conceal the lights, and given that the core is plywood of almost surely varying thickness, the prospect of coming out with a perfectly balanced panel is doubtful. It might be a satisfactory and less expensive solution than starting from scratch.

From contributor F:
I agree with the assessments as to the root of the problem. I have one more observation. In reading your website, you mention re-sawing the 4/4 stock and finish planing to 9/16". At that point, how long did you let it acclimate before laying-up? The movement you see, or at least part of it, may be the natural response to the milling process. That was exacerbated by gluing up in a cross-grain situation, i.e. face gluing to plywood. In addition, you used a glue that uses moisture absorption to cure – this could be a problem considering the quantity you would have had to use. An epoxy or polyurethane or urea based adhesive may not have shown the problem to such an extent.

As far as mitigating,is there any chance you can get away from a gloss finish? You could either rub to a satin finish or sand and strip back and use an oil finish. This probably won't do anything about the root cause (except for an oil – this might allow the top to breathe) but at least lighting would not highlight the problem.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor J:
If you decide to redo the top completely, I have to agree with the others suggesting you re-saw the IPE and apply it as a veneer. If you decide to go this route, it would be wise for you to use either Unibond 800 or an epoxy type glue to attach the veneer. These types of glues are appropriate for veneering. Also, keep in mind that one of the cardinal rules in veneering is to apply a like hardness and thickness veneer to the other side of the ply or MDF. You must have a balance of pressure on both sides to achieve a proper sandwich that will stay flat. If you apply veneer to one side only, it will cup and twist.

As to thickness, I think you could put on a 3/32 inch veneer and then sand it to around 1/16" finished. By doing this the veneer will still take a beautiful finish and also be thin enough to withstand years of use. Gluing the thick (5/8") IPE to a ply backing with Titebond will always cause any board to cup. Titebond and other glues of this type are not good for veneering as they move too much and will eventually lead to other problems such as the joints opening as the veneer shrinks and expands. You might want to look into the archives at this forum about veneering if you decide to pursue this route.