Epoxy
By Adam (RED) Weston
Over the 12 years I’ve been in this hobby, I’ve tried to use some of the most modern building techniques available. One item that a lot of people use but perhaps not to its greatest extent is epoxy. Actually, epoxy isn’t all that modern, its been around since the 60’s, my grandfather has been a pattern maker and pioneered some of the techniques for using epoxy resin in his industry. He likes to tell horror stories about having to have exactly 85% humidity in the room and getting the combination to stay at 115 degrees while you’re mixing it... fortunately our epoxies are a lot more resilient these days.
Let’s start with what is epoxy, well in general, there are two parts to epoxy, the resin and the hardener. The resin is made up of very long molecular chains, the hardener is a catalyst that allows these long chains to make side branches that connect the chains to each other all along their sides. This chemical reaction can give off allot of heat, especially if the mixing ratio isn’t right. It’s important to get the specified ratio right for the epoxy, most of what modelers use is a 1:1 ratio. If you are off by too much, the bond isn’t as strong as it could be and usually ends up rubbery, or never sets up. I got some of those Mentadent tooth paste dispensers that Bob Lamb showed off a couple of months ago, they look like their going to work pretty well. You might give them a try, but he was right, don’t over fill them, you might get epoxy all over your work table, but I wouldn’t know about that.
Now because of how epoxy sets up, it makes for some great applications in modeling. You can bond virtually any porous surface to any other, and know almost exactly how long it will take to set up. The two important parts of that last statement: you only know how long it takes to set up if you get the mixing ratio close, and the surfaces work a lot better if they are porous. So next time you want to join the faces of two sheets of ply try scraping up the surface with a knife, or a nail, or anything sharp, do the same for plastics or metals, it really helps the adhesive bond.
Now the modern builder has a choice of several great products for bonding wood etc., when should he/she use epoxy instead of CA? CA is truly wonderful stuff, and if you want to keep it light, and we all do, CA is a must, but the problem with CA is that you generally only get the strength of the contact area in your bond. This means, you don’t get a fillet between two butt joints, unless you use some kicker and that tends to make for a brittle joint. So there is one good place for epoxy, doing your fire walls and other essential joints, especially the ply to ply ones. Here’s another place, epoxy tends to be very tough once it sets up, and doesn’t run or bleed when it contacts fuel etc. So you can fuel proof cowlings and fuel tank areas with it. One trick to keep this light, try thinning the epoxy with either alcohol, or xylol, don’t mix in too much, you can get way to much alcohol quit quickly, I like to get the pure denatured stuff from the hardware, but rubbing alcohol works too, although the water content will make the epoxy a bit cloudy but wont hurt the joint. With that bit of knowledge, I just told you how to clean up all you epoxy implements as well. Just get out the alcohol and wipe down everything. I store mine in an old saline solution bottle (well labeled), or a spray bottle.
Another way to thin the epoxy is to heat it up, now be careful with this one, the more you heat it the faster it kicks over (sets). One slick little trick is to put some epoxy on the fire wall etc. and then hit it with the heat gun and let it soak into the wood. This trick can also be used to get that stubborn last drop out of the epoxy bottle, either heat the bottle in the microwave or with the heat gun. Try to avoid open flame and epoxy, it will burn and smells really, really bad. Your significant other or the land lord may ask you to go sit in the corner and we wouldn’t want that. Heat also works to break down cured (set up) epoxy, this is great when you are rebuilding something but not great if you have fuel proofing too close to your engine. Epoxy is a plastic, a thermoset plastic, which means once it sets, it can’t be reformed with heat. Your typical canopy type plastic is a thermoplastic, which can be heated and reshaped. This heating trick can work to speed up working times, if you are in a hurry, try making a make shift oven, I like using a cardboard box and a spot light or heat gun. This can speed up curing times and won’t effect the joint, although it can make the epoxy run out of the joint.
Another use for epoxy is for fiberglassing. Fiberglass is very strong, and also rather light, if it is done correctly. Like the name says, fiberglass is a number of tiny glass fibers that are made into threads and then coated with a substance that makes them adhere to epoxy well. The typical builder will use fiberglass (or just glass) to add strength to the center section of a wing, or around a fire wall. There are several different weights of glass and they are described by their weight per square yard. Depending on the weight of glass, different types of epoxy can be used, and different lay-up techniques are appropriate. The typical center wing joint on your typical trainer is 6 oz cloth. To do this joint, I will typically use 30 minute epoxy, and then thin it with alcohol so I can brush it on. I’ll then mark the area where the glass will go, brush on a thin coat of epoxy to the wing sheeting, lay down the glass and then brush on some more epoxy until the glass is soaked through and looks uniformly wet. I sometimes do the top and bottom in one piece and let the ends hang off the trailing edge, then let them cure and sand the hanging off part flush. Although sometimes I’ll do it in two pieces and wrap the bottom or top, depending on looks or how well the wing saddle fits around the leading edge and then overlap the top or bottom, respectively. Here’s another good spot for that home made oven, but watch the dips and runs.
Another technique for doing wing centers, I picked up in Connecticut from my buddy Rich Ippolito. In this technique, we use three or so layers of 3/4 oz cloth. This makes for a very clean and light center section, but takes a little more effort. The technique is similar to how the scale guys finish there planes for painting. I will first cut my glass pieces. Depending on the span of the plane, where cut outs are, where the open bays start etc., I will cut the first piece. For a fully sheeted 60 size plane, I’ll make the first sheet 18 inches wide one for each top and bottom. The next pieces I’ll cut 8 inches wide and the last about 6 or so. I could put another piece maybe 12 wide in if I thought I needed the strength. I then take two full rolls of paper towels and use these as wing cradles. If you’re like me I buy the big 8-packs of paper towels and I leave two rolls on my table for wing holders. While you’re at it, get a third roll out for later. Next, I take a soft bristle paint brush and brush the widest piece, using the static to get the piece to lay down. Then I’ll mix up a batch of clear epoxy paint. This stuff is really thin, like water, and smells really good too. Now I use a different brush (not as soft as the previous one) and paint the epoxy onto the glass. Let it soak in and then do the other side in the same manner. Flip the wing back over, put on a second coat of epoxy. Then flip it again for a second coat on the other side. Now, take the next narrower piece and lay it on the wet layer. Brush on some more epoxy, flip the wing and do the same to the other side. Now the final layer, do the same, lay it down and brush it on. This may take two coats, I usually don’t put on enough, but I am much too weight conscious too. Now, take that extra roll of paper towels and rolllll it along the glass and soak up the excess epoxy. This can be done with finishing epoxy, but I haven’t used that, you might have to thin it, give Jack Brewer a ring on that one. Let this cure, and unfortunately, if you have any rough spots, its best to leave them since you can sand right through the glass and this is bad for strength. I usually just give it a 240/320 grit smooth over and then a light coat of coverite before covering. Watch the iron heat, once again too much heat will break down the epoxy.
Now let me chat about the types of epoxy. The typical modeler uses the 5 minute or 30 minute varieties. These times are the approximate cure times, depending on room temperature and mixing ratios. I generally mix these type of epoxy on a piece Plexiglas, using a 1/2 wide metal spatula. I make it a point to stir for a minimum of 30 seconds and try to not miss mixing any. Always mix smaller batches, the less epoxy you mix up the slower it will typically set up, and plan what you’re going to do before you start squeezing the tubes this saves time and wasted epoxy. Clean up is once again with alcohol, but if you let it dry, just use an wood chisel and scrape off the Plexiglas and spatula. I have mentioned the epoxy paint and the finishing epoxy, these are similar except I mix them in a tuna fish can, or maybe the top off a Tide bottle. Be careful with the epoxy paint, it has some acetone in it and will eat a lot of plastics, i.e. Styrofoam, clear plastic cups. To measure mixing ratios with these, I have some tiny measuring cups that look like the ones off of cough syrup bottles, or I also use syringes (without the needles), but they’re tough to clean up.
Another type of epoxy we see in hobbies are really thick structural types, like hobbypoxy formula 3. This type is thick enough to set up a fillet without running. Essentially its is plain epoxy with lots of filler, usually microballons, or fiberglass slivers. Both of which you can add yourself to your 5 or 30 minute epoxies. Now to get these into the joints is sometimes a challenge, either lay up too much between the pieces and let it ooze out, or try this trick I read about in the Northeast Sailplane Catalog. Take your epoxy with the filler already added, and put it in a ziplock bag. Now twist the bag so it looks like a pastry icing device and clip the corner to just the right size for the bead you want. Presto! This method makes for good wing saddles too, cover the wing with some wax paper, with maybe some parting agent on it, lay on a big bead of epoxy and then screw the wing down, and finally use a rounded spatula to get the right curve. This is definitely an art, and not something I’ve had a great deal of success with. If it doesn’t look good, get it off with alcohol before it sets up.
Another type of epoxy is what most of the full scale aircraft industry uses for laying up fiberglass and carbon fiber, laminating epoxy. This is 12-24 hour curing epoxy that typically requires some screwy mixing ratio (i.e. 5:1 or 2:1) and is really strong, but expensive (a gallon cost me $80, actually not to bad, if you don’t mind planning your glue purchases 4 years in advance). I use it for sheeting wings, and laying up fiberglass. If you think you have a use for it stop by and I’ll sell you a mustard jar worth of it (I have 7/8’s of a gallon left after a year of building) Mixing this stuff, I use my triple beam balance, or the little mixing cups.
So, if you’ve read all of this, you are probably thinking, now, I must know everything possible about epoxy, but of course you don’t. The biggest trick I’ve found is try it before you fly it. If you want to try epoxy somewhere where you’ve never seen it, test it on some scrap first (once again stop by, I have 7 kit sized boxes full of scrap). Then compare it to the old way of adhering it. Maybe we’ll set up a demo of some of this stuff for an upcoming meeting!
RED