When working with epoxy, there are a few best practices worth considering:
Make sure the surfaces to be epoxied are clean and fresh. Once cleaned of any oil and surface contaminants, and possibly abraded etc, do not touch! Finger oils greatly reduce bond strength!
Rubbing alcohol is not useful for cleaning as it contains an oil additive. Oily solvents are also not useful for a final cleaning. Denatured alcohol can be used to clean up after oily solvents (turpentine, mineral spirits, acetone, some others).
If there is moisture in what you are bonding, the epoxy will not work right.
If there is alcohol or other solvents in what you are bonding, the epoxy will not work right.
If the surface is polyurethane or teflon or many other materials, the epoxy will not bond well. Some such surfaces can be pre-treated to improve the bond. Pre-treatment can consist of passing the active part of a flame over the surface to be bonded, briefly. You'll cause a bit of chemical reactivity to form at the surface which allows the epoxy (or other adhesive or bonding agent) an improved shot at getting a good bond. The reactivity fades pretty fast so work fast!
If the temperature is below 60F it is too cool. Consider treating 70F as a minimum temperature. If the temperature is over 90F consider it too hot. Too cool a temperature and the epoxy won't cure well. Too hot and it cures too fast and becomes brittle. That's why a proper cure cycle for structural epoxy may have a cure temperature, and a higher post-cure temperature cycle, rather than just trying to cure at the higher post-cure temperature.
Weigh your epoxy, or measure out by volume. Don't eyeball it.
Mix thoroughly, until the epoxy looks clear. Scrape the sides and bottom of the cup, and the mixing stick, repeatedly. Rotate the cup around periodically while mixing.
Do not whip the epoxy to mix. That is just adding air and is counter-productive. Air bubbles do not add strength.
When you think you are done, scrape the epoxy out into a second cup and discard the first along with the mixing stick. Start mixing again with a new mixing stick.
Do not thin the epoxy. Slight additions of solvent cause substantial loss of structural properties.
Now apply the epoxy.
Keep the rest of the epoxy you don't use, or at least a sample of it. Pour some out on some wax paper or some other suitable surface. Use the sample as a cure test. When the epoxy snaps when bent a moderate amount, then it is mostly cured. At least, it shouldn't creep under briefly applied load. In other words, it is now safe to handle. But full cure if elevated temps and/or a post cure cycle are not used, is on the order of a week.
For elevated temperature cure and post cure, to improve mechanical properties and thermal properties such as Tg, see manufacturer's data sheet. If a manufacturer's data sheet is not available, it is not structural epoxy and likely should not be relied upon as such! But that is of more concern for high performance Black Rock projects than for the run of the mill high power rocket.
There's a lot more when it comes to additives and fabrics, but this is a start for general bonding work.
I don't know if any of this helps, but there are plenty of people using bad practices with epoxy. Because epoxy is pretty forgiving (particularly typical consumer epoxy), most of the time people get away with lots of bad practices but don't realize how much of the possible final structural performance they are losing in the process.
Also different epoxy is designed for different jobs. West Systems for instance is designed for fabric coating wooden boats. It is thicker so it doesn't run as much on vertical and near vertical surfaces. It has a very low Tg (glass transition temperature) so the epoxy creeps on hot days. Wood changes dimensions based on humidity and temperature, so having an epoxy which adjusts is a benefit. For an airplane wing however, that same "adjustment" could be very bad... West is very good for what it is designed for. It is not good for some other applications. I'm just using West as an example because it is well known and often mis-used.
Gerald
