Q & A: Does Polyurethane Glue Fill Gaps?
I hate it when I slip up and make a joint that doesn’t fit
tight. I heard that polyurethane glue will fill gaps and hold a
loose joint together better than yellow glue. Is that true?
The largest gap that polyurethane glue will adequately
bond across is about .005 in. (That’s pretty small.
The cover of this magazine is .004-in. thick.) The outside
limit for yellow glue is about the same. Polyurethane
glue will expand in a gap larger than .005 in. It foams and
forms very small bubbles of carbon dioxide gas, hardening
into the equivalent of styrofoam. These bubbles may
“fill” the gap, but they form a very weak bond. Filling and
bonding aren’t the same thing. Polyurethane glue has
many good properties, but bonding very loose joints
isn’t one of them.
Use epoxy when parts don’t fit well. It dries by chemical
reaction rather than the loss of water, so it doesn’t
shrink. Neither does it form bond-weakening bubbles.
Epoxy will both fill and bond a gap as large as 1/16 in.
This story originally appeared in American Woodworker December, 2001.