Why is My Resawed Wood Bowing (Or Cupping) from WoodCentral
The poster who began this discussion was having problems with some old, 8/4 oak. It was thoroughly dry, but it still gave him problems when he ran it through his band saw. - Editor
"Last night, I was resawing 8/4 oak boards in half and, just when the band saw blade was through the entire board, the two pieces popped apart. Both sides bowed away from the direction of the cut. To make it clearer, if I put the two pieces back together, the ends would touch, but in the center they were about 1/2" apart over a 48" length. Before anyone asks, these are really old, very dry pieces of wood. I got them from Grizzly in the late 90's, and they have been sitting in my shop, which is heated and air-conditioned, since then. I cut apart six of them last night, and it happened on all of them. I have another 6 to do. Could it be from the heat of the blade? I'm using a 3/4" blade in the saw, and I'm feeding the wood at a normal rate, not overloading the saw at all. This happened before when I resawed some pieces from this lot. I can't understand it. Any insight into this would be helpful. I have the boards clamped flat to my workbench, but I hold little hope they'll straighten out. Any advice on how to straighten them would be appreciated, too!" - Ed
One respondent had quite a bit of insight, including that what had actually happened was "cupping," rather than "bowing." - Editor
"There is stress locked into this lumber. It is common with thick oak because oak must be dried very slowly and sometimes it isn't. Because you see it on most all of the lumber, it is a drying defect, not stress as a result of some aspect of how the tree grew. This sort of stress can only be relieved by steaming the lumber, a process that will soften the fibers and let the lumber relax.
"If I understand correctly, Ed was in fact resawing the board into two thinner boards and the resulting thinner boards cupped. Cup is across the width of a board; bow is along the length. Cupping can result from two processes that can be distinguished by how quickly it occurs. If the resawed pieces cup immediately after resawing, it is a result of the lumber being stressed. The most common cause of stress is drying, (either air or kiln) the lumber too fast. When dried too fast, the interior of the lumber is under tension. When resawed, this tension pulls the thinner boards into a cupped configuration. It is very easy to dry oak too fast because of how it dries and the requirement to dry oak very slowly. Stress can also occur from how a tree grew, but this isn't the cause of Ed's problem.
"If the lumber cups after a day or more, it is because the lumber is gaining or losing moisture from one side faster than the other. There may have been a moisture gradient across the thickness of the thick piece. Or the resawed lumber may have been stored in a way where one side gained or lost moisture faster than the other." - Bill T.
He also received some advice on ways to go about fixing the problem. - Editor
"Try stacking and stickering the boards as soon as you finish resawing them. Be sure there is some air movement through the stack. Give it two weeks and see if there is any improvement." - Howard A.
"Depends on how the wood was sawn into lumber, its drying profile (air dried? in what conditions, kiln dried? too fast, too hot, etc.), and what's happened since (RH cycles, storage, etc.) -- so many factors. For older oak and a few other species, I take skim passes over the jointer and set it aside for a couple weeks or more (ideally) before resawing." - David B.
"White oak can be a lot tougher to dry than red. The cellular structure of white oak makes it tougher for water to get out. That said, in that amount of time, it should not still have had drying issues. It can depend on the type of tree as well. If it was a leaner, it can have what is called reaction wood. There are lots of stresses in reaction wood. I would stand the boards up so air can get all around it and see if it calms down over a week or so." - Dick C.
"My advice is to save this for when you need 8/4. And go buy some more 4/4, when you need it. I don't think you are going to wrestle this wood back to flat, without reducing it to 1/2" on the jointer, turning most of it into shavings. It probably will be fine for projects where you need 8/4, if you use good techniques, like sawing oversize, letting it settle, then straightening it at full thickness, or taking equal amounts off each face. It is harder to find thick oak here sometimes, but I can always get 4/4.
You are just buying yourself headaches doing this." - Keith N.
And some elaboration on the method of eliminating the cupping via steam. - Editor
"You can build a steam box really easily, just knocked together out of CD plywood with a pipe or hose going to a steam kettle. Hose has to be able to resist the heat of the steam, and should be fairly short to keep from losing all the heat. If you use metal pipe you can super heat the steam by applying a torch to the pipe (saw a neat demonstration in the 8th grade; teacher burned paper with steam by doing this) You also may not need to remove the bow, depending on how bad it is and what the boards will be used for. Table tops and casework are far more forgiving than door panels. From what I've read about steam bending, you shoot for 1 hour per inch of thickness - - I'll admit to having never tried it, but straightening a board would be the same principle as steam bending, you're just clamping against a straight caul rather than a curved one." - John
"The wood must become plastic to relax the stress. This will occur as soon as it reaches a moist heat temperature of something near the boiling point of water. Actually, it will occur before this temperature, but I don't know exactly what temperature and it will depend on humidity. So be sure to sock the steam to it until the wood temperature reaches about 100 C. Then, of course, the wood needs to be carefully dried so as to not reestablish the stress. The wood will dry quicker than it did the first time because the cells are already largely collapsed and the wood will not hold as much water as it once did." - Bill T.
Protective Finish for Plywood Shop Floor from WoodCentral
The woodworker who began this discussion had a question about one of the very foundations of the shop: the floor. Specifically, whether and how he should paint or otherwise finish it, for both practical and aesthetic reasons. - Editor
"I have about 1,100 sq. ft. of bare plywood flooring that I need to do something with before any shop equipment goes in. This needs to happen two weeks from now when I have a week off from work. Plywood is that crazy heavy 9/8" stuff. Walls and ceiling are drywall and painted white. Lighting is good. The building itself is somewhat high-end -- brick and stone outside, metal roof on top.
The objectives for the floor are to (1) avoid ugly, (2) make it cleanable (not just sweepable but to keep any tracked-in mud out of the wood), and (3) avoid dark, light-depleting colors. I guess old-school would be to paint it gray. I'm wondering about using a water-based poly such as used on hardwood floors. The ply is not pretty, but perhaps it is better looking than industrial gray. Maybe a different color paint is advisable. Is there such a thing as "floor grade paint" Ideas? Opinions? Any 'I did this' advice?" - Ron
He did receive several "I did this" types of advice. - Editor
"I built a shop/shed about seven years ago and I put epoxy paint down, and then covered it with the old carpeting and padding when we recarpeted the house. Of course, I went around the cabinets and such so it would be more comfortable on the feet and it was free. But if I didn`t have the carpet, I would have stayed with just the paint. I didn`t think it was slippery when I didn`t have the carpet." - Larry
"Depends on your expectations. When I built my shop, I intended to paint the plywood floor, but I only got as far as priming it, and it has held up pretty darn well for lo these past 12 years. Sweeping is quite easy, and the light color of the primer does brighten the shop, although it's not as aesthetically pleasing as the medium blue-gray paint I had originally intended. My reference to your expectations had to do with how pristine and protected you want this floor. Most shop floors get a lot of glue and finish dripped on them, depending on your work habits. I only ever worried about the most egregious of these. If you want a floor that can be mopped clean, you probably want floor enamel or epoxy garage paint, or you could use polyurethane over a coat of regular paint." - Ellis W.
"I have had very good luck using Ace Hardware polyurethane paint on my concrete basement floor. It's very tough and they will mix any color you want. A very light gray would be my preference for a shop floor. Light enough to provide good reflective light without the glare of a white floor." - Paul
"My floor is concrete. I used a paint with epoxy fibers. It has held up well. My friend Mike has an 'Advantek' floor (plywood). He painted his floor with a similar product. It worked well for him. He ended up tinting his paint a different color." - J.L.
And some research and theory. - Editor
"1.According to my lighting engineer brother in law, painting the floor a light color will lessen the need for more lights overhead. 2. Some sort of finish will aid sweeping. 3. Oil-based porch enamel is the hardest, most wear resistant, of the paints. It takes a week to cure to full hardness when it is warm. It leaves a finish slicker than I would want on a shop floor. This paint plus sawdust could be a hazard. There may be some epoxy product that would be better. 4. Other than the disadvantage of color, floor varnish is a good choice. I was told by a paint chemist that the oil-based products are more durable than the latex porch and floor paints. I got mine at Glidden and painted the deck of a flat bed trailer with it. It took two weeks to cure, but it cured hard and has held up outdoors." - Bill T.