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End Grain Top Cupping12/30/14
We glued up a Walnut end grain top that is 3" thick, 54" wide, and 96" long. We first glued together random width 8/4 lumber into 2' to 4' wide panels, then cross cut these into 3" strips. We glued these strips together staggering the joints until we had 3 sections 18" wide which we later sanded and glued together into the 54" wide top. The top is resting across 3 sawhorses but will cup both the long way and width way. It is not finished on either side yet. When installed the top will have a 18" overhang on one side and end. Any suggestions on how to keep the top from cupping?
What type of glue was used and how long has the piece been drying?
Titebond 3. The lumber was kiln dried and the top has been glued for a week. It is under a heater inside the shop.
The heater is drying out one side. Are the edges turning up towards the heater?
That would indicate the top is drying more then the bottom and shrinking lifting the outer edges.
Yes, the edges are turned up to the heater. I think the heater is doing a lot of the cupping although it did get worse over the weekend when the heater was not on. I just hope that it will not do this installed in the house with the heat blowing on top of it. I will move it away from the heater in the shop and see how it improves.
Flip it over under the heater and wait til it's almost flat, then remove the slab. Put it in a place that offers equal air exposure around the whole slab. When you finish it you will need to do equal coats on both sides. If you are going to be using a film type finish Coat one side and as soon as you can coat the other to keep it sealed.
Having unequal air exposure to the slab may cause it to warp again. And once you put the coating on the 2nd side the movement will slow to a crawl or stop. It may get frozen in that shape.
If you are just going to put an oil type finish on it then there shouldn't be such a rush to get both sides equally coated. But I would still do it anyway.
If you are coating with a film coating, do the back (unseen) side first. So when you flip it and the finish is still a bit soft, any marks really won't matter that much.
What type of heater?
Radiant heaters will cause more issues than forced air because they heat exposed surfaces rather the ambient air.
It is radiant heat. We have done a number of solid wood edge grain tops but this is our first end grain. Also installed this heater about the time we completed this end grain top. First time really having an issue with this but I can understand the problem. The customer was wanting to apply the finish themselves but now I think it would be in our best interest to do it ourselves.
We sometimes have stacks of glued up panels in the shop. If they are hanging around for awhile we dry stack them. Even at that the top one always cups. We flip it daily and this seems to work to keep flat untill finishing.
To extend Joe's suggestions: placing a simple sheet of corrugated cardboard or ply wood on top of the stack will prevent cupping.
To the OP: move your top away from the heat and use 1/2" thick sticks to hold it above a bench or sheet of ply. Then put a few 1/2" sticks across the top and a sheet of 1/4" ply or equal and let the top acclimate and flatten out. Then finish it.
Real world old guy tale: I once ran a shop that, among other things, prehung 3 houses of interior doors a day. These were almost all 1-3/8" solid w pine 6 panel doors from a good maker. As we grew, the storage for these doors moved out into unheated space. We kept about 350 doors on hand, so they were organized in flat stack racks and picked one at a time with a forklift and specialized skid.
After the move, we started getting a large amount of doors that were warped. These we kept and returned for credit when the manufacturer delivered the next load. We went from about 1% to over 10% warp rejects. I started looking around and first found that many of the ones rejected as warped were not warped. They were set aside in a return area, and had good air circulation on all sides. I then made observations on the storage and saw that the top doors bowed up in the center (along their height) on the really wet days and they bowed the other way - ends up - on the really dry days. This was on the top door only in every stack. They all moved the same way, at the same time.
I used a sling hygrometer and measured 3 times a day and also correlated that info with the amount and type of movement in the doors. My boss, who referred to me as a 'college type' behind my back, was impressed that my efforts were just like 'that science stuff'.
The fix was easy: we sized a piece of Masonite to each door size and placed it on top of the stack. It was slid aside to pick a door or two and replaced. The reject rate for warp went to 0 immediately.
The cause was the changes in humidity in the 14,000 s/f storage area. The area had two 20' wide overhead doors that were closed at the end of the day and opened in the mornings. The RH could change 30 points in a day. As I now know from Dr Gene and others - wood moves in response to changes in moisture content brought on chiefly by changes in relative humidity in the environment around the wood. One side of each door was exposed to these swings in humidity and the wood does what it does - responds.
I'll agree with all of the cause and effect due to RH / MC and think besides doing the finishing, that I would first really saturate it with a penetrating epoxy. I have been using it on some of the exterior millwork to add to the longevity, due to it's moisture exclusion. The stuff is extremely thin, and penetrates really deep. It will make the surface a lot harder, and the finishing that follows a lot easier too.
But I still have some concern for the 18" overhang on the front and end. That would be nothing on the ends for along the grain, but with the short grain, I'd be pretty worried, especially if there was any sapwood left on. It will absorb the moisture out of the air or spills much faster than heartwood, putting it in compression, which will put the heartwood in tension. It also desorbs moisture much faster, so it causes problems going both ways.
With the grain so short, I'd be worried if I were ask to do this job in this manner, and be pushing for some kind of support brackets supporting that long overhang. If it was designed by an architect, I'd want it in writing that it is their problem if it fails, not mine.
I think Jamestown Distributing has the CPES on sale now, but I don't know for how much longer.
A late response: you should check the moisture yourself, as the term "kiln dried" has no precise meaning so the wood may have been kiln dried but not down to 7.0% MC.
As implied, the only reason that wood shrinks or swells, including cup, is because it's moisture is changing. Certainly having the same humidity on both sides is good and also a finish can help, but the real key is getting the correct initial MC.