|For years, the only type of epoxy I
used was the "five minute" variety sold at most hardware stores. It
was great for quick repairs -- to mend a cracked plate or fix a broken toy.
It wasn't until I discovered epoxy "systems" that I realized how
useful epoxy can be for woodworking. It's extremely strong, virtually
waterproof, and doesn't shrink at all.
Like the five minute variety, an epoxy system comes in two separate parts: a
resin and a hardener. When the two parts are mixed together, a chemical
reaction occurs that hardens or "cures" the epoxy.
What makes an epoxy system special is you can vary the curing time by choosing
the type of hardener. Fast hardeners cure in about ten minutes. Slow hardeners
offer a much longer assembly time -- up to an hour. This makes it the perfect
choice when you need to glue up a project that has lots of parts.
In addition, epoxy does something other glues can't. It will bond dissimilar
materials -- like wood and metal. We often use it to glue a bolt in a jig. Or
to repair a stripped out screw. And since epoxy only requires that the parts
touch each other for a good bond, it's the perfect solution for those awkward
situations where you just can't get a clamp on something.
But the thing I like best about epoxy systems is that they allow you to
customize the epoxy to fit your application. By varying the mix ratio on some
systems, you can change the curing time. Other systems let you mix in fillers
that can change the consistency of the epoxy.
The only drawback to using epoxy systems is that they can be expensive. And
since they come in two parts, they're not as convenient as pre-mixed glues.
There are three basic steps to working with any epoxy system: measuring out the
two parts, mixing them together, and applying the mixture.
By far, the most important step is measuring. When measuring out the parts,
make sure you follow the directions. Some manufacturers allow you to vary the
mix ratios. Others don't, and they warn that an improper ratio can result in a
mix that won't cure, or one that is weakened, even if it does cure. The most
typical mix ration is two parts resin to one part hardener. But depending on
the product, it can vary up to five parts resin to one part hardener.
You'll want to measure only what you think you'll need because you can't save
the leftovers. If you're doing a large job, it's better to mix several small
There are a couple of safety tips to keep in mind when working with epoxy.
First, avoid skin contact -- wear gloves. Second, epoxy fumes can be hazardous.
So make sure there is adequate ventilation. And wear a respirator and eye
Note: If you do get epoxy on you skin, don't use a solvent such as thinner or
denatured alcohol to remove it. Solvents only drive the epoxy in deeper.
Instead, use a waterless hand cleaner (like Goop) used by auto mechanics. Once
you have measured out the two parts, your ready to combine them. Stir the
mixture well, scraping the sides of the container often.
Note: Don't use Styrofoam cups. One of the by-products of the chemical reaction
is heat (see below), and Styrofoam cups can melt. Mixing takes anywhere from 30
seconds to five minutes depending on the amount you're mixing. You might want
to write down the time when you started mixing the parts so you'll know how
much time you have.
Before applying the epoxy, make sure the parts to be joined are clean and free
of dirt and dust. If you're gluing wood, we've found that freshly planed
surfaces work best. If you're gluing metal to wood, it's a good idea to
slightly roughen the metal before applying the epoxy.
To apply the epoxy, we use a disposable glue brush. The secret to a good bond
is to apply the epoxy to both surfaces. This ensures that the joint won't be
starved -- that there will be plenty of adhesive for a strong joint.
Once the parts have been coated, all you have to do is press them together and
hold them in place long enough for the epoxy to set. You don't have to apply a
lot of pressure. So this can be done with string or tape or even rubber bands.
After the parts are immobilized, it's important to scrape off any squeeze out
with a putty knife while the epoxy is still slightly soft and doughy. Once the
epoxy has cured it's difficult to remove -- and it's tough on your tools.
Personal Story: A couple years ago, I was building a wooden boat with a couple
other guys. We were gluing up sections of the mast with epoxy. One new guy
volunteered to mix up a batch -- and he got the ratio wrong. As he stood there
holding the tuna fish can stirring the two parts together, he suddenly yelled
and threw the can in the air. What happened was the chemical reaction between
the two parts created so much heat, it burned his fingers. What I find really
interesting here is that the epoxy in the can was hard as a rock BEFORE the can
hit the floor.
Go to Tip #30