|09-13-2014, 10:55 AM||#1|
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I am: MartinZ
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EPO Foam Rebuild Techniques F20
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I am sure that anyone involved in EDF flying has had some experience with the foam models. The vast selection of models available, makes them an inexpensive way to get started in the hobby or just increase your fleet. Also due to the reduced weight, most models perform very well, particularly if you upgrade the EDF units. The only problem with the foam models is that they do not hold up like conventional balsa and fiberglass models. This brings up the question what is the best technique to repair them. Having suffered a few repairs myself this past flying season, mostly due to hand launch techniques for my smaller EDFs, I have had some experience working with EPO foam. With my most recent rebuild, I thought it would be a good opportunity to start a new thread on repair techniques for foam models. Hopefully this will create some interest and other inputs from many of you that may have other techniques.
My most recent experience with EPO foam involved the rebuild of a Great Planes F20. As you can see from the photos, one of our club members turned this model into a lawn dart. The aircraft was pretty much destroyed from the wing LE forward, wings torn off and main fuselage bent. This was the second crash for this model and it appeared it was destined for the garbage can. Being comfortable working with foam, I took it on as a challenge to see if I could rebuild these chunks of foam back into a flying model.
1. The first step was to take all the chunks of foam home to see if all the pieces were there. With the exception of a few joints pulled apart everything appeared to still be there. The next step was to try to expand all the crushed and bent pieces back to their original shape. It is absolutely amazing how this type of foam retains it original form and shape once moist heat is applied. This is easily done with the use of steam from a kettle or boiling water. I must admit that my wife was not too impressed when she saw the front section of my model sticking out of her new stainless steel pasta pot. The fuselage was a little more difficult as the centre of the top deck was bent and required heat to straighten it out. The kettle worked well for this but dam those new kettles with auto shutoffs. The only problem with the heating process is that it causes the foam to slightly expand and loose the smooth exterior surface, taking on an alligator like appearance.
2. With all the pieces back to their original shape it was time to start gluing everything back together. For those seams that broke off clean with no soft foam around the joints, foam safe Cyano worked great for this. For those areas that did not fit tight, I used 5 min epoxy. At the high stress locations such as next to the wing saddle, I inserted sections of tooth picks into both pieces when joining them together to strengthen the joint. I have used this technique on larger foam models using small carbon fiber spars with great success.
3. With the basic airframe back together it was time to begin the reshape process to straighten everything out and remove the alligator skin surface. Any major dings or dents were filled with expoxy/mico balloons. I then used 100 grit paper and a sanding block to straighten out any variations. The only problem with this is once you start sanding the foam, it does become some what fuzzy. Once all the sanding is done, I use Formula 560 white glue to coat all the exposed foam. This is a canopy glue but adheres great to foam and dries clear. I just rubbed it with my finger over all the exposed foam. Once the glue was dry it was time to switch to 220 grit wet to sand the glue off.
4. Now the creative part starts. If you want a really nice finish you can put a coat of primer down first or just spray on your favourite paint to get you back in the air quickly. I sprayed a coat of the cheapest primer on, (Armor Coat from Canadian Tire). Once dry any small dings or dents were filled with spot putty (automotive) and then reprimed. The putty works well provided it is put over a solid base and applied thinly. If you put it on an area where the foam is flexible, you will get cracking through your paint finish when the foam flexs. Once I was happy with the finish I applied a coat of Krylon paint to the whole model before adding the Humbrol camo color. I wanted to use my new air brush. When it comes to painting EPO foam I have found that pretty much any paint can be used as long as you don't spray to close to the model (allows the thinner in the paint to evaporate before reaching the model). Always try a sample first. I had a bad experience on my HK Harrier that I tried to spray Krylon on. It began melting the wing but it was not EPO foam. At the end of the process all the gluing and painting only added a few ounces to the weight of the model.
When it comes to EPO foam models I have come to the conclusion that they are somewhat indestructible and if you have a catastrophic event and have all the pieces, anything is repairable.
Last edited by SARPLT; 09-26-2014 at 07:58 AM.
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