Woodsmith #142-polyurethane-glue.pdf

T OOLS &

T ECHNIQUES

P OLYURETHANE G LUE

When it comes to building any out-

door project, you want to use an adhe-

sive that stands up to the weather.

Here in Iowa, that can mean lots of

rain in the spring, humid heat in the

summer, and dr y, frozen winters.

I’ve tried a number of different

glues on outdoor projects in the past

(plastic resin, resorcinal, and epoxy).

But this time, I decided to use one

that’s relatively new: polyurethane

glue. (This glue is readily available at

woodworking stores and catalogs and

many home centers.)

When you hear “polyurethane,” you

probably think of the popular finish-

es first. But polyurethane glue has lit-

tle in common with these finishes. In

fact, it doesn’t even have much in com-

mon with other outdoor glues.

For one thing, the outdoor glues

I’m used to working with are all two-

part concoctions. But there’s no mix-

ing necessar y with this glue, it’s ready

to go right out of the bottle.

APPLICATION . But more importantly,

polyurethane glues cure differently

than other glues. Instead of curing

by evaporation or by a reaction that

takes place when two chemicals are

mixed together, these glues simply

react with the moisture in the wood

(which should be at least 8%). To be

on the safe side, I usually add a little

water to one of the workpieces, mist-

ing it on with a spray bottle.

I apply the glue to the mating work-

piece. Here again, polyurethane glue

requires a different approach. With

other glues, you don’t want to “star ve”

the joint (not apply enough glue). But

polyurethane expands as it cures, and

the tendency is to apply too much. In

fact, figuring out how little glue to

apply takes some getting used to. I

found it easiest to spread the glue thin

with a plastic spreader, see photo.

This way, I was sure the wood was

covered without too much excess.

WORKING TIME . With most other

glues, things start to get a little fran-

tic as I tr y to get the clamps in place

before the glue sets up. That’s anoth-

er reason I like using this glue.

There’s no rush. You have 20-30 min-

utes to get the clamps in place and to

make sure the assembly is square.

Of course, the down side to this is

you have to leave the clamps on a

while longer. I typically like to keep

them on anywhere from 4-6 hours.

As I mentioned earlier, these glues

expand as they react to moisture. But

“foam” may describe it better, see

inset photo. When I saw this bubbly

mess oozing from the joint line, my

first reaction was to wipe it off right

away — before it hardened. But actu-

ally, polyurethane glue is much easi-

er to remove after it has dried. Unlike

other glues, this glue will scrape or

sand off easily. (I like to use a cabinet

scraper.) That’s because the glue

doesn’t cure rock hard. You can even

dent it with a fingernail.

DRAWBACKS . Polyurethane glue def-

initely deser ves a place on the wood-

worker’s shelf. But don’t leave it there

too long. After the bottle has been

opened, the glue has a shelf life of

about 6 months to a year.

That’s not too unusual. Other glues

have a limited shelf life too. But

polyurethane glue is a bit more expen-

sive than other glues. So when buy-

{ One unique characteristic of

polyurethane glue is that it

foams as it reacts with the

moisture in the wood.

ing some for a project, I tr y to get just

what I think I’ll be able to use.

There’s one more drawback to

note. Because this glue reacts with

moisture, it will react with the mois-

ture in your skin, so it’s a good idea

to wear gloves when applying the

glue, see photo. And even though

polyurethanes have little or no sol-

vents in them, you should have ade-

quate ventilation, too. (Also, if you

have respirator y problems, be aware

that these glues contain isocyanates.)

So what’s the bottom line? For most

projects, I’ll probably still rely on

trusty old yellow glue. Polyurethane

glue is a little too pricey to replace it.

But I’ll definitely be using poly-

urethane glue again — on my next

outdoor project or when I need a lit-

tle extra assembly time. W

Woodsmith

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