Replies

1. 
   HowardAcheson | Apr 09, 2003 07:54pm | #1

   Bet you used PVA (yellow) glue. It's one form of creep and the reason many do not use PVA glues for panel glue ups. RJ Slainte posted a good explanation some time ago. Maybe he will repost it. If not, I will post it tomorrow.

1. 
   HowardAcheson | Apr 11, 2003 07:59pm | #3

   RJ Slainte has not shown up so with attribution to him, here is his prior post:

   **********

   Creep has been known about for many years, perhaps even centuries. It's nothing new and has already been defined precisely, so no need to reinvent the wheel here. PVA is the classic and renowned creeper.

   The tendency of the glue to ooze out of joints is one form of creep. A classic example is in a slab edge to edge glueup, such as a table top. When ambient humidity rises the timber and the the glue swell. When the ambient humidity goes down the timber shrinks again, and so does the glue, but the glue doesn't all shrink back into its original place resulting in a line of pimples disfiguring the finish. Actually, under sustained high humidity my anecdotal observations are that the glue keeps on absorbing moisture and creeps out of the joint without the need for the timber to shrink. The symptoms can also be seen sometimes at the shoulder line of other joints such as mortise and tenons.

   Another cause that I've witnessed several times is to make a solid timber slab with edge joints in a fairly humid workshop without climate control using PVA as described before. Then right away get to planing and preparing the surface ready for polishing with scrapers, sanding, etc.. Right after that apply the polish, whether it be varnish, pre-cat, or another finish, and take the piece into a drier house, either heated or air conditioned. Three to five days later the rows of pimples will be apparent as the timber shrinks.

   Somewhat related to this is using an aid for edge glue alignment, such as biscuits with PVA. These jobs need time to settle. If you are in too much of a hurry and prepare for and polish it all as soon as you get the piece out of the clamps, the chances are you'll see a row of rugby ball shaped depressions telegraphing the position of every biscuit a couple or three weeks later, although I've seen them appear as much as a year, or more, later. The moisture in the glue swells the wood either side of the biscuit, and it takes a week or two for this moisture to work its way out of the joint properly. My advice then is to not rush into the preparing for and polishing stages after this type of glueup, particularly table tops and the like which show off every flaw. I don't use PVA for this specific kind of glueup at all anymore here in hot, humid Houston. I tend to reach for the hide glue nowadays, either out of the pot, or the liquid stuff. Slainte, RJ.

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1. 
   SgianDubh | Apr 12, 2003 02:28am | #4

   I hadn't spotted this thread, Howie, and those do look like my words, ha, ha.. They describe one form of creep, and a related phenomenom-- the ability of biscuits to telegraph. Here's a brief description of another form of creep to watch out for, which has nothing to do with the original question in this thread.

   Creep can manifest itself in the form of stressed glued joints sliding past each other. A good example is to form a small radiused curved laminated bend using PVA where the bent part will tend to straighten out due to the glues elasticity-- it never goes off absolutely solid.

   An acquaintance of mine made a sofa with laminated rails using PVA. He ran a groove in the top edge of the rail paralell to the glue lines to accept the metal tang of rubber seat webbing. A year or two later one of his friends fell through the sofa. Upon inspection it was found that the groove worked for the tang had become a rebate (rabbet,) ha, ha.

   In other words, glued joint lines under a stress where the stress is paralell to the glue line can move and slide apart. Slainte. 

   Website                                   The poster formerly known as Sgian Dubh.

1. 
   HowardAcheson | Apr 13, 2003 04:54am | #7

   Richard, I was involved with a large shop a few years ago and we had many problems and call backs when using a commercial formulation of PVA glue in panels. We spent lots of time with the tech people from the supplier and never did resolve the problem with PVA adhesive. We substituted DAP/Weldwood Plastic Resin adhesive which is a urea formaldehyde formulation.

   Suppliers we used for preformed bent laminations all used this adhesive because of non-creep characteristic and it high strength. This is what I use now for panel glue ups but continue to use PVA for other applications.

   You are correct that PVA is a real risk for bent laminations or applications where there is continual shear pressure on the joint. It is the biggest negative with PVA. PVA will slip.

   Forest Girl, the adhesive I mention is a good choice. Poly adhesives also have no creep

   Edited 4/12/2003 9:58:32 PM ET by Howie

1. 
   forestgirl | Apr 13, 2003 06:30pm | #8

   "...real risk...where there is continual shear pressure on the joint."  Howie, thanks for that detail.  It answers the next question I had about this problem.  Now I'll know when to stay away from PVA.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl   ;-)

2. 
   forestgirl | Apr 12, 2003 03:09am | #6

   Howie, thanks for posting RJ's earlier description.  Is hide glue the main alternative to the PVA?  Is there a second alternative that works well?forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl   ;-)

2. 
   CAGIV | Apr 11, 2003 08:34am | #2

   There are two very good curved "stair guys" over at breaktime,  If you would like, go over there and post your question and make your title something to effect of "curved stair question"  it will be sure to get their attention.

   View ImageGo Jayhawks..............Next Year
3. 
   Armin110 | Apr 12, 2003 03:04am | #5

   Biff,

   I want to buy you a beer, heck I'll buy you two beers. I have been carping about the glue line phenomenon, as pertaining to handrails, for several years. Unfortunately most trades people finish up a staircase and move on before the problem occurs, then it's out of sight and out of mind. The first dozen stairs I built were laminated using Titebond, no matter how many times I sanded it down after time the slight bumps would show up. Unlike most other woodwork, handrails get fondled all day long, and after time the unevenness would detract from an otherwise awesome looking project. I finally switched to Unibond 800, a catalyzed resin glue and never had a single problem. The key is, titebond does not produce a rigid glue line and after time the problem you described will surface. The other advantage to Unibond over Titebond is it sand beautifully, will not clog sandpaper and is less prone to show up as a splotch during finishing.

   http://www.northernsunwoodworks.com
4. 
   biff | Apr 20, 2003 08:17pm | #9

   Thanks to all who helped me out with the PVA glue lines and recommendations to eliminate them in the future. I've been away for 10 days but wanted to say thanks still.

   Biff