Tuesday, March 29, 2011

Tech Tip Tuesday

Don't be afraid of the Goo...

I've said it before and I'll say it again - students seem to be intimidated by working with epoxy. It's not as scary as it may seem. I think the biggest concern that most people have is that it is going to set up before they're ready for it to, or that it will not set up at all.

First, let me say that while I've posted an image of West System's 105 epoxy resin, there are many good epoxy manufacturers. West System is one, System III, Raka, MAS and others all make good epoxy. Choose one that is appropriate for your use. If you're building a canoe that you want to have a clear finish on that shows off the beauty of the woo, the clarity of the epoxy is an important choice. If you'll coat with a colored paint, this isn't such an important feature of the epoxy. Recently, my students have been working with West's 105/207 resin-hardener combination with good results.

Before you ever start working with the epoxy you should prepare. Read the manufacturer's instructions carefully. Get proper protective equipment including gloves, barrier cream, goggles, and organic vapor/particulate masks and/or great ventilation. You want to avoid exposure to the resin so that you will reduce your potential for sensitization to the epoxy. Get good materials to clean up after your epoxy - white vinegar is good for cleaning up uncured epoxy and acetone (some vendors recommend their proprietary solvents...) is good for cleaning up cured epoxy. Avoid using acetone or other solvents on your skin as they can drive the epoxy through your skin and into your system. Disposable wipes and paper towels should be handy as should drop cloths to protect surfaces that you don't want to have spoiled with epoxy. Set up a workstation with the supplies you will want - epoxy and pumps or other metering equipment, mixing cups and sticks, fillers, reinforcements, tape, scissors, razor blades, rollers, squeegees, brushes (foam or chip bristle - but if you use bristle, trim them short to make them stiff and apply epoxy to the base of the bristles near the ferrule to retain the bristles) a waste basket and spare gloves - in short anything you think you might want. Wear clothes you don't value much, or buy Tyvek (tm) coveralls - you will likely get epoxy on yourself.

If at all possible, work with items which are ultimately disposable if they will come in contact with epoxy.

Are you ready?

Nope. Not quite.

If at all possible, you probably want to work with a friend and plan your work. One person should be applying the epoxy, the other should be supplying them with mixed resin and materials as required. It really reduces the panic potential. Have a plan worked out for who will do what ahead of time - and whatever you do, don't let the person mixing the epoxy get interrupted and mess up the mix ratio. If you want pre-cut pieces of glass-cloth reinforcement, or bias cut strips, prepare them now.

You should both be prepared with an almost Zen-like calmness. There is no reason to rush and panic - you should be working within the temperatures that the manufacturer recommends for the resin system you've chosen. There are some things that will let you have more working time - don't leave mixed resin in a mass in a cup - put it in container that will spread it out a bit or apply it to the boat to slow the process down a bit. Keep in mind that typically for every 10°F rise in temperature, curing time will be cut in half and for every 10°F drop in temperature, the curing time doubles.

Are we ready yet?

Nope. Not yet.

We need to make sure that our work environment is correct. I know that there is no ideal workspace, but you need to do the best you can. It should have good lighting, be dust/dirt/fur/bug free, have good ventilation, out of direct sunlight, no animals, children or other distractions, be 70-75°F (for most resins, but again - read the manufacturer's instructions) with the ability to control temperature.



You've got to make a decision about how much time you and your assistant will have to work with the epoxy at a session. If you've got limited time, you'll need to let the epoxy set between coats (which requires a proscribed time/temperature combination for the specific epoxy system), and prepare the surface per the manufacturer's recommendations prior to applying subsequent coats. Typically this includes washing off amine blush with some form of liquid cleaner (usually soap and water and/or ammonia and water) and then lightly abrade the surface to provide a mechanical bond - 3M's Scotch-Brite (tm) is a good way to clean and abrade at the same time. We usually use denatured alcohol to wipe the hull down after that to assure that there isn't any residue left.

Ideally, I would have all boats we work on hot-coated. Because of the limited amount of time in my classes, we don't usually "hot coat" as there isn't enough time. I've discussed this before. This is because the hot-coating process provides for both a chemical and mechanical bond between the layers of epoxy. The downside? It takes a whole day (usually about 14 hours or more) to do this for the outside of the canoe.

We also usually put a sealer coat of epoxy on the hull before applying cloth - this helps minimize bubbles and "starved cloth" which is weak, structurally and not cosmetically pretty. When applying epoxy to bare wood, the ideal situation is to warm the work space up to heat up the wooden hull - to say 80°F or more. Then turn the heat off and allow the hull to cool as you apply epoxy - this avoids off-gassing of the hull which makes bubbles and improves epoxy saturation of the wood. If you try to epoxy in the sun, the sun will heat the wood, driving off gasses and giving you MILLIONS of bubbles. Not good.

After we've got the hull saturated and have prepared the surface, we drape the hull with fiberglass cloth. Fiberglass cloth has a coating - sometimes Volan (tm) which is a water-based coating that help the cloth saturate with either epoxy or polyester resin. You don't want to handle the cloth with bare hands and get oils from your body on the cloth which may prevent good saturation. Wear your nitrile gloves. Also, keep the cloth clean and dry to avoid loss of the coating and loss of transparency. To cut the cloth, it is a good idea to pull a single thread from the cloth's edge where you want to cut to give you a mark to cut along. This isn't as hard as it sounds, but requires a bit of practice to avoid breaking the 'glass yarn in the middle. Roll or "flake" the leftover cloth and store it so that it stays clean and dry and doesn't crease.

Smooth the cloth over the hull and work out any irregularities by carefully "massaging" the cloth with gloved hands. Avoid snagging the cloth on sharp edges.

Now we're ready to apply resin.

We measure out the proper ratio of resin/hardener and mix for at least 1/2 a minute being sure to get into all of the corners of the container to properly mix the epoxy. While we do this, we avoid mixing in excess air - you're not trying to whip the epoxy. Only mix an amount you are comfortable working with - if you mix too much, you're likely to waste it. You can always mix more, you can't mix less.

Apply the resin to the hull starting from the middle of the hull and working outwards methodically, avoiding wrinkles. The goal is to get all the cloth wet out so it is transparent. Pay attention for dry spots and bubbles. The most difficult thing to do is to apply epoxy to vertical surfaces with a squeegee - I usually hold the edge of the squeegee near the sheer against the hull and pour a little resin on the hull just above the squeegee and pull upwards towards the keel. Don't panic at this point - I tell students that it usually takes about 3 or 4 canoes before you really get comfortable with the process. I'm joking here, but not much.

If the epoxy you're working with starts to heat up or gel - stop using it and put it aside. You'll only make a mess if you try to use it.

When the hull is wet out, we go back, starting at the center of the hull and use the squeegee to remove excess epoxy by working from the keel towards the sheer with light pressure. This is because the cloth tends to "float" on the resin adding weight, but not strength. You should be able to see the texture of the cloth when this is done properly, but without white areas that indicate resin "starved" cloth. We wipe the excess epoxy off the squeegee using a paper cut with a vertical slit in it about 2" down from the rim. When the hull gets to the "green" stage - just starting to cure - it can be trimmed at the sheer and stems and reinforcements added at the stems and wet out. This is much easier than when the cloth is fully cured and hard.

Subsequent coats of epoxy can be applied with your choice of brush, roller or squeegee until the cloth is "buried" - avoid sanding the cloth which weakens the actual composite and ruins the transparency.

Not that hard, really, it just takes some patience, knowledge and practice.

While you're working, avoid contact and work as neatly and cleanly as possible. If you spill, stop and clean up - you'll only be spreading uncured resin everywhere. When done, clean up and throw away the items you can. One caveat here - avoid throwing away large amounts of mixed resin as curing epoxy is an exothermic reaction (it generates heat!) and can cause some real problems when it gets hot enough to smoke, melt cups and could potentially cause a fire. Dispose of carefully!

No comments: