How Hard Is Your Epoxy? By James Goss      Like most modelers I use a lot of epoxy and I like to keep a good supply in stock. I never know when I will need eight or ten ounces to sheet a wing or to repair a crashed plane. How long will epoxy keep? This is a question I hear a lot and I now have a better answer for it. I have been using Tower epoxy for many years and have had good luck with it until the last few years. I think the problem is that I try to store it too long in advance before I use it. About two years back I received an order from Tower for five 9-ounce sets of epoxy resin and hardener. When I received the order I discovered that the resin was as hard as a brick, of course Tower made it good. Since then I have got several other epoxy orders from Tower and the resin would go hard in just a few months. Several times I simply discarded it instead of being bothered with sending it back. Tower's epoxy has a date on the bottom that recommends it being used by that date in time. It is easy to keep the epoxy stored far beyond this date before you have a need for it, especially if you are not in a building cycle during this time. When you go to get the epoxy off the shelf you are out of luck if it is hard.      This happened to me just last week. About a year back I was getting ready to sheet some really big foam wings so I ordered twelve 9-ounce bottles of epoxy. I have always liked to use small bottles instead of getting the epoxy by the gallon because it is easier to handle. A little while after I received the order I began some home remodeling and was out of my shop for about six months. I finally got caught up on my home project, or at least to a resting place, and decided to build planes again. The first thing I needed was some epoxy so I went to my epoxy storage and grabbed one of the twelve bags. You guessed it; the resin was as hard as a brick. I pulled another from the shelf and it too was hard. All twelve had gone bad and I thought I was dead in my tracks. In the past I have actually used some of the hard resin to make a mix and it did work ok, but the resin was still a past and not completely hard like this batch was.      This time the resin was so hard you could not press the plastic bottle by hand. The resin had formed a crystallized lattice covalent bond that was so tight the resin would break like glass before it would bend. There was no way to use it in a mix. It is always the resin that gets hard and never the hardener. I was getting ready to dump all twelve bottles of resin when I had an idea. I have always used the microwave oven to warm resin and hardener when it was cold and hard to get them out of the bottles, so why not try the microwave. Trial and error showed that it takes about 30 seconds in the microwave for a full 4.5 ounce bottle of resin. After it is heated allow about 10 seconds to react and then squeeze the bottle for about 10 seconds. The crystallized resin will return to a liquid and it is good as new.  It works every time and reactivates the resin no matter how hard it has become.      This has given me a new outlook on storing epoxy on the shelf for many years. I don't think the hardener will ever go bad, but I am not 100% sure. The epoxy I used in this experiment was 30-minute epoxy. The method will work on any epoxy, slow or fast, because the resin is the same regardless of its setting time, only the hardener is different. I hope knowing this will help your modeling experiences as it has mine.