Replies

1. 
   forestgirl | Jan 15, 2004 08:04pm | #1

   The carpenter is correct.  As a general rule, you want both sides of the wood to be finished so that they will react to humidity equally.  If your case has been in a very stable environment, there might not be any noticeable change, but if it's subjected to a humidity swing you could see problems.  Why am I hedging with "might" and "could"?  How much change you see will depend on the type of materials, size, etc.  Tell us more.  But plan to at least give a couple coats of shellac to the inside, or better yet use the varnish.

   forestgirl Another proud member of the  "I Rocked With ToolDoc Club" .... :>)  you can take the girl out of the forest, but you can't take the forest out of the girl ;-)

2. 
   JohnWW | Jan 16, 2004 01:04am | #2

   Gal,

   The problems with an unevenly finished board will only show up when the relative humidity of the air around the piece changes significantly and the wood starts to absorb or lose moisture relatively quickly. 

   You don't say where you are, but where I live in New England the indoor air has been bone dry for a couple of months and won't become more humid again until spring, so problems caused by an uneven finish wouldn't show up around here until April or May.

   Whether or not you will have a significant problem is also dependent on the species, dimensions, and grain orientation of the wood in the piece combined with the ability of the joints to resist the warpage. 

   To be safe it is better to finish both sides the same if possible.  Note that even if the board is finished equally on both faces the wood will still gain and lose moisture through the finish and some warping may still occur, but an even finish on both sides will avoid creating additional warpage that could damage the joinery.

   Hope this helps, John W.

    

1. 
   Gal11 | Jan 16, 2004 01:21am | #3

   Thanks Forestgirl and John,

   I should then finish the piece from the inside also, an unpleasant work for me since the case is already finished, but he who does not work with his brain works with his hands :-)

   Gal 

1. 
   VeniciaL | Jan 16, 2004 01:55am | #4

   Gal,

   Whoa. Don't rush to finish the inside of the piece. What is the case construction? Solid wood? Plywood? Frame and panel? Thickness? Dimensions?

   The advice that wood should be finished on both sides is true (generally). It's also true that wood will exchange atmospheric moisture more rapidly on the unfinished than the finished side.

   But that does not mean that a case structure needs to be finished inside and out to prevent warping. Many (most) are not and stand the test of time. A thin panel will warp if finished on one side. But it's unlikely that a properly constructed case will ever be subject to that kind of stress. It depends a lot on answers to the above questions.

   Finishing the inside of case goods is usually a design decision and has to do with the overall look and feel rather than mechanics.

   VL

1. 
   JohnWW | Jan 16, 2004 02:43am | #5

   Venecia,

   Good points and a good counterbalance to my posting.  I just presumed solid wood construction, which would be a worse case scenario.

   John W.

    

2. 
   SgianDubh | Jan 16, 2004 03:37am | #7

   Venecia, I think you're trying to say that an enclosed cabinet, i.e., one in which the drawers and doors, etc., are kept closed most of the time experience a different micro-environment to the general environment experienced by the outside of the cabinet. Therefore rapid changes in relative humidity due to turning on the heating or air conditioning will generally affect the exterior of the cabinet more rapidly than the inside.

   A film forming polish applied to the outside, resistant to changes in general or absolute air humidity equalises the comparitively stable interior of such a cabinet left unpolished.  Slainte.Website

1. 
   VeniciaL | Jan 16, 2004 04:01am | #8

   Richard,

   No, actually that's not what I meant to say. But now that you've said it that way . . .

   Um. . .

   Yeah.

   VL

2. 
   SgianDubh | Jan 16, 2004 04:39am | #10

   Venecia, on the other hand, let's say you have a table top-- maybe a Regency style tilt top with a bird-cage underneath and bearers radiating from the approximate centre across the grain. Here, both sides of the table top experience changes in the humidity in the air-- absolute, relative, whatever.

   Then we add the complexity of applying a 'finish' of some sort to the top face, but ignore the bottom face, and a couple of hundred years later we wonder why some of those tops remain flat, and some don't? Meanwhile, the upper surface has received lots of well meant attention, sunlight, and wiping off with damp rags, etc., and the bottom face has been ignored.

   I don't think there's a cut and dried answer. Slainte.

   Website

3. 
   joinerswork | Jan 19, 2004 08:49am | #14

   Sgian,

   I agree with what you've described, but think its probably more important to consider what are the construction methods used.   That is, traditional case construction techniques were developed to counter any tendencies toward warping--if a solid wood case is dovetailed together at the corners, it's not likely to be able to curl up.  Likewise, a frame and panelled case, when the frame members are not overly wide, yet stout enough to resist the panel's tendency to warp.  If the frame is stout enough, and the panel has room to float through swell-shrink cycles, things will stay flat.  I've seen many an old chest, few if any were originally finished inside, and they hold up quite well.

   Cheers,

   Ray

3. 
   Tao | Jan 16, 2004 04:18am | #9

   Venecia,

   I should have read the posts more carefully, since it was clear you had already made the point I was trying to illustrate. I apologize.  All I saw was a bunch of posts saying you have to finish both sides..........

   SD did sum it up pretty well, didn't he.

   Michael R.

4. 
   Gal11 | Jan 16, 2004 01:54pm | #11

   The case is for a street organ, which will mostly play at home. It is made of massive birch wood. The measurements are: 23" width 14" depth 17" height. The case is made of frame and panel but only the sides (17"x14") the front and back are open frames. The frames are about 0.6" (15mm) thick, the panel is about 11mm. The lid is already finished from both sides 3 coats of waterborne varnish with acryl and polyurethane resins, the bottom is screwed to the case and is actually part of the organ and not the case and is not finished at all. All the other sides of the case are also finished with the same 3 coats of this varnish but only from the outside.

   I have cut out all the wild grain wood and used only the very white "boring" parts of it.

   Even though the front and the back are open, the front will be full of pipe which will actually block most of the air from coming into the organ, and on the back I also have a board screwed on (for easier access to repair the organ).

   About the humidity where I live, now I live in Germany where it is not very humid, but there is a possibility that I will move to a much more humid area, so I want to take this fact into consideration also.

   I hope this information is detailed enough.

   Thanks for your help 

   Gal

1. 
   VeniciaL | Jan 16, 2004 09:05pm | #12

   Gal,

   Yes, you do need to put finish on the inside of the box. The panels must be finished on both sides. Since the cabinet is done, you will be finishing the inside of the frames at the same time.

   It's not necessary to put varnish on the inside, although you could. If you do, it might be easier to use a diluted varnish and apply 2 coats. Lay the cabinet on one side, varnish the inner surface of that side and wait an hour or so before turning the cabinet to do another inside surface. That way, there will be no varnish runs or drips.

   You could also use shellac, which I recommend. Use a 1 to 2 lb cut. It will dry so fast that you'll be ready to turn the cabinet as soon as you're done a side. You can reasonably put 2 coats on all the inner surfaces in an hour or two.

   Warping should not be an issue when you're done. If you have left enough room in the frames for the panels to expand when they absorb moisture in a humid climate, the frames will not crack. Or if the panels are plywood, there will be no problem.

   VL

2. 
   Gal11 | Jan 18, 2004 11:30pm | #13

   Thanks for the advice!!

3. 
   Tao | Jan 16, 2004 03:32am | #6

   And now for the other side of the story:

   I worked in the furniture manufacturing industry for over 20 years, and have also done a lot of antique restoration off and on over the last 30 years, including a lot of pre-industrial pieces from prior to 1840.  In all that time, I have seen very few pieces finished on the inside where it doesn't show.  Hoosier cabinets are one exception; a lot of them are painted white inside.  Usually the only part of the inside that is finished is the part behind glass doors, or the bottoms of drop leaves.

   Film finishes can be a good protecton against liquid water, but most of them only retard the movement of water vapor. They don't stop it.  All solid wood expands and contracts with changes in relative humidity, finish or no.  You have all seen that, I'm sure.

   Over 40 years ago, I was taught the same thing about finishing both sides, but a lifetime of woodbutchery and observation have led me to believe that if wood is going to warp, it's going to warp, finish or no.  There are exceptions, of course.  Leaving a glued-up panel on a concrete floor, or exposing one side to the hot sun will cup any panel, but that can usually be straightened right out.  Cupping and end checking usually result from wood that was not in the equilibrium moisture content range when it was machined or glued up.  That was probably the biggest cause of reject parts in the factories I ran, even though we were pretty diligent about checking every incoming shipment for moisture level, and we controlled the humidity in the factory.

   Other sources of warping are reaction wood, and case hardening from improper drying.

   That's my experience from making over 2 million board feet worth of (mostly solid)wood furniture a year for the better part of 20 years.

   None of the above applies to balancing veneers and laminates.  That is an absolute necessity MOST of the time.

   FWIW

   Michael R