Raised glue lines after curing
A glue line that is raised days after parts are assembled and the glue has cured is a sure sign of wet lumber. 1998.
by Professor Gene Wengert
Q.
Several days to a week after I glue up two pieces of wood with PVA adhesive, it appears that the glue line has oozed out--that is, there is a bump at the glue line, even though I sanded it smooth shortly after the glue had "set." Am I using the wrong adhesive?
A.
The glue itself becomes quite rigid after 24 hours in most cases. In fact, that is the cause of the problem. The wood pieces that you glued together must be a little too wet, either at the time of gluing or after you have sanded them. The wood then subsequently shrinks down a little bit--as a rule of thumb, 1% shrinkage for 2% MC change for oak (a very high shrinking wood) to 1% shrinkage for a 4% MC change for pine and other lower shrinking woods.
However, the glued area resists shrinking because the glue adds rigidity. The glue joint area stays the same size, even though the wood around it is shrinking. Hence, the bump that you see.
The cure is to wait a little longer before sanding and make sure that the wood MC is correct. Refer to the simple chart of RH vs. MC (below). Most houses and offices are at 30% RH in the fall, winter, and spring throughout most of the country.
RH MC
0% 0%
30% 6%
50% 9%
65% 12%
80% 16%
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
Click on Wood Doctor Archives to peruse past answers.
If you would like to obtain a copy of "The Wood Doctor's Rx", visit the Wood Education and Resource Center Web site for more information.
Would you like to add information to this article?
Interested in writing or submitting an article?
Have a question about this article?
Have you reviewed the related Knowledge Base areas below?
KnowledgeBase: Adhesives, Gluing and Laminating: Glues and Bonding Agents
KnowledgeBase: Lumber & Plywood: Storage
KnowledgeBase: Wood Engineering: General
KnowledgeBase: Wood Engineering: Wood Properties
KnowledgeBase: Woodworking Miscellaneous: Woodworking
KnowledgeBase: Knowledge Base
Would you like to add information to this article? ...
Click Here
If you have a question regarding a Knowledge Base article, your best chance at uncovering an answer is to search the entire Knowledge Base for related articles or to post your question at the appropriate WOODWEB Forum. Before posting your message, be sure to review our Forum Guidelines.
Questions entered in the Knowledge Base Article comment form will not generate responses! A list of WOODWEB Forums can be found at WOODWEB's Site Map.
When you post your question at the Forum, be sure to include references to the Knowledge Base article that inspired your question. The more information you provide with your question, the better your chances are of receiving responses.
Return to beginning of article.
|
