OK. We've never done this before, but, let's see where it goes...
This article was written in 1996, and edited today to delete some non-relative information.
There may also be some newer materials out there since this was written, so if anyone has any additions, please chime in...
Ward-O
IN the BEGINNING
by Ward Van Duzer
Email WEVD@aol.com
OK! You’re ready to start assembling that kit. Uh-oh, you didn’t buy any glue? Well, maybe we should talk about that a little before you leave for the hobby shop. In the “good old days” there was Ambroid. And that was good. Ambroid on the motor mounts. Ambroid on the nose. Ambroid the ribs and leading and trailing edges. Ambroid the stab and rudder. Even Ambroid on the cloth hinges. Life was simple. And that was good.
But 2008 is gooder folks! We have cellulose (Ambroid, etc) cements, aliphatic resins (yellow, white and brown), sandable aliphatics, polyester resins, fast and slow epoxy resins, finishing epoxies, and CA’s (slow, medium, fast, and friendly). If this is all gooder, it’s certainly considerably more confusing. In order to clear up some of the mysteries we’re going to use some vocabulary that must be understood by all of us to mean the same thing. So, let’s group some of these words and describe then as they apply to model building.
Hard, brittle, crispy. These generally all mean the same thing. Glues with these qualities will crack under extended vibration. Never use near the nose or stab! Real hard to sand smooth.
Flexible, bendable. Not as hard or crispy as CA’s, The slow cure epoxies are good for use in the nose and stab areas.
Soft, flexible, sandable. Will stand up to vibration and twist, and won’t ball up when you try to sand it.
Cure. A chemical process that does not require air to dry, or bond the cement.
Now, let’s try and clear up some of those adhesive questions. And present some of the problems as well....
CELLULOSE CEMENTS... Ambroid, Duco and the like. These require the acetone in them evaporate to effect their “drying”. Therefore, these are not a good choice for doublers. The acetone may not ever fully evaporate through the balsa sides. These cements also dry rather brittle and are not a good general choice for any area subject to continued vibration. I also question their ability to repel alcohol based fuels. When using these it is often best to smooth the cement to both parts to be joined and allow it to dry before finally cementing the parts together. This is the “double gluing” process you may have heard some old timers talk about. These cements sand fairly well, but are best left overnight to “fully” dry. I have placed the word “fully” in quotes to note that these glues will continue to dry for years! When ALL of the acetone and thinners have evaporated they leave nothing behind but cellulose! Ask those who watched me flick the ribs out of my 25 year old framed up Barnstormer!
EPOXIES... These “cure” rather than dry, making them the best choice for nose doublers. (Nothing has to evaporate)! The slow cure (at least 30 to 45 minute) epoxies will maintain some softness to them therefore they will maintain the flexibility you need around the nose of your ships. Great for use in bonding hardwoods (plywood, maple motor mounts, and landing gear blocks) to balsa. Properly cured, epoxies are 100% fuel proof. Quick cure (5 minute) epoxies are OK for field repairs and the like, but should (generally) be avoided. These cure before they have a chance to penetrate the wood, and the resulting joints are not as strong as the slow cures. Finishing epoxies should be thought of as just that! Finishing surfaces only. These are quite thin when mixed and therefore quite easy to work with, BUT, they cure very hard (therefore, brittle) in order to make them sandable for finishing purposes. I have seen these recommended by some for gluing doublers, but I won’t use them for anything except FINISHING! They just weren’t designed for strength. They ARE good for fuel proofing (finishing?) the inside of your tank compartments.
One final note about epoxies. Beginners typically will think that “more is better. We can/should use the glue to fill voids and gaps.” WRONG! Epoxy glue itself is not very strong. It is it’s ability to chemically bond materials that gives us the qualities we love about it. And, very little actual glue is required to achieve that purpose. Spread it thinly on two surfaces, then scrape it off (both surfaces) with a playing card, and you have enough to do the job. Too much epoxy adds POUNDS to your airframe, and removes DOLLARS from you wallet.
RESINS...No, Polyester Resins are definitely NOT the same thing as epoxy! Polyester resins are most often identified by the hardener coming separately in a tiny little tube or bottle. You add a few drops to several fluid ounces of resin material. Although not used readily in our phase of the hobby, these are used rather frequently in R/C construction. Resins will glue some plastic materials that epoxies won’t. They may be used for fiber glassing noses, and painting tank compartments. They will sand better than epoxies on F/G noses, etc. But, beware. Resins are not compatible with epoxies! I am told that epoxy resins contain wax, and the polyesters HATE wax. You cannot put polyester resin OVER epoxy (resins). They will not cure!!!! Now, some folks tell me they have had success putting epoxy over poly resins. Let us remember that Epoxolite IS epoxy!
CONTACT CEMENT... Often used for securing sheeting to foam wings. This is the “once it is down, it’s STUCK forever” stuff. Quick and easy, but beware... it gets heavy fast! If you are going to use these (not recommended), use them SPARINGLY! Slow cure epoxies (used sparingly) are probably a better choice for this application.
ALIPHATICS... These would be my first choice for general construction. But don’t put this article down yet! There is good news/bad news with the Aliphatic Resins! Let’s get the “COW” and the “CHAIR” stuff out of the way first. The white stuff Elmer sells is used by some to sheet wings with. I’ve not used it for this so I really can’t say from experience, but I’ll stick with epoxy. (Ooooooh!) I’ve heard it said that the water soluble aliphatic cements will lose their strength in humid or rainy weather. These also have the very undesirable characteristic of rolling up in little balls when you are trying to sand your glue joints smooth. The Brownish-Yellow, furniture version of this glue will ball up as well, although it is a stronger glue. This glue also seems to set up quicker than Elmer’s. But your first choice should be found in the hobby shop! This is not one of those places where you can chintz by going to Sear’s, or Ace Hardware! We are talking about several SANDABLE ALIPHATICS that provide all of the good characteristics of the “White or Brown Cows”, along with sandability. There is something to be said for a gap filling glue, that gives you time to work parts into final position before it sets. Spend the money and get a brand you can trust. Since these generally do not come in container that allows you to apply it in the small amounts you’ll want to, you’ll need to buy a glue gun (or two or three). Fairly quick cure, (1-3 hours).Especially good for kids to work with too! These belong in your shop!
CA’s... Cyano Acr*&@@?+$$$ ! Yup! We all use them in some form, and love them. They have revolutionized the model world. But why have I saved them for last? Because they are the best? Hardly!
Maybe I am a dinosaur, but I have learned not to trust the CA’s for all applications. That’s why I made you read through all of the information about the other materials first. OK. What’s BAD about a cement that let’s you build a model overnight? First let’s understand that the words GOOD and BAD are relative terms. Got that? GOOD!
“Instant” is the word that best describes the CA’s. This seems to fit the bill for those individuals that have the need for instant gratification. (Will patience be lost forever?) There is, however, a dear price to be paid for “instant”. (And 15 to 45 second cure is still “instant” to this old goat). At least in our glues. The penalty is Hardness. Brittle! Cracks.Completely un-sandable. Someday you need to take a wing assembled with CA and twist it back and forth gently from the wingtips. It sounds like 15 microwave ovens loaded with popcorn going off, right? Well folks, that popping and crackle-ing is the glue cracking and breaking! Think about that! Is this how you want your next stunt beauty to be held together? Yes, there are times, and airplanes, where this kind of assembly may be appropriate, but it is definitely something you should think about before building your next “Sooper Swooper”.
No. This does not mean that I don’t use CA’s anymore. It is a great substitute for pins! I use it to hold parts in place while the real glue dries, or to hold parts in position while I apply the real glue. I’ll use it on longitudinal joints where there is little or no twist. I.E., in cementing leading edge sheeting to the “C” or “D” tube spars. But to build an airplane with...Nevermore! And never, never use CA anywhere near the nose of any ship you build. Or, on wing center sections. Unless you really want an ornithopter for a stunt ship!
Topic: Which adhesive (glue) to use and where or not. (Read 4818 times)