Hopefully you are able to find some useful information on this page.
REINFORCE THE FOAM
Works on Depron and EPP, other stuff too. Around landing gear, motor mounts, and under batteries can sometimes use a little help. Often if your plane is painted or even inked these spots could use some more bonding area as the glue might peel up the paint. Create a stronger surface, or prep for bonding by smearing on some hot glue (low temp) Sounds ugly, but start by laying down some thin beads to establish a pad. Keep the tip down and do some small circles or go back and forth, moving across the area slow enough to keep the glue melted. The goal is to create a thin layer of glue that is very well embedded into the base material. The gun tip can be used to squeegee the glue, just takes a little time. You can also remove excess by doing this and wiping it off with something else.
While EPP can make for a very durable R/C plane, one thing seems to plague them. Hinges! While many foamy manufacturers are cutting EPP kits with hinges built in, they still break. This tip work just as well for Depron.
If you want to add strength without adding lots of weight try this. Get some packing tape with fiberglass filament in it. Using a razor cut the tape into strips with a single strand of filament in it. More filaments are fine, but not generally needed. Put a thin bead of hot glue (low temp) where you will be adding the tape. I like to go about 5/8″ on each side of the hinge line. Smooth the glue out by putting the tip of the glue directly on the bead and rubbing it some without adding more glue. It is ok to let the glue cool before putting the tape down. With the tape in place rub the glue gun tip over the top of the tape to reheat the hot glue under it. You could optionally use fishing line. Cut a slot into the foam about 1/8″ deep 1″ on each side of the hinge line. Now simply use the tip of the glue gun to press the fishing line into the slot, squeeze just a dab of glue as you go. Once it’s in place trim the line and you’re done. I typically do this only near the outboard edges of the hinge, and around the landing gear.
This is not much of an issue here at Light RC anymore. Part or becoming a supplier of EPP sheets is that I can demand better quality than the RC marketplace had previously. If you are still getting your’s somewhere else then read on…
Epp has many great features, and a couple not so great. Main thing that can really suck is if a couple of parts that have different thicknesses. When you get EPP panels or parts that do not have the same thickness at the glue joint, try this. Using hot glue (low temp) put a decent bead right down the center of the glue joint on one part. Not to much so as to squeeze out, but enough to stay warm for a few moments and spread around both parts where they join. Quickly push the parts together on a flat and smooth surface. As soon as they are together start pushing the parts down against the surface. Push directly on the point where they meet, making sure that the thicker part get squeezed. Keep moving around (think spider playing the drums), both your fingers and the part(in case a little glue got out underneath). Make sure that you keep the parts pushed together and keep pushing down. Most of the time this can make a bad fit look great. It will always make for a better bond between parts
PROTECT THE FOAM
Most foamy designs use heat shrink tubing to build control arms. Avoid burning your foam buy putting up a shield. I like to grab the trusty razor saw, it’s steel, thin, easy to prop up, doesn’t slide. Anything steel can be useful, even a kitchen spoon. Try to stay away from aluminum, it transfers heat too fast to be of much good.
The steel will allow a very reasonable amount of time to heat the heat shrink. You can also use the tool of choice to act as a wind break. Very helpful when at the field and making a last minute adjustments to your linkage (providing you are using adhesive lined heat shrink tubing).
Gluing in spars can sometimes be a messy thing. Getting it wrong can mean a very ugly mark on the plane, spars coming loose, or creating warped panels. Depending on the type of materials, as well as the design of the panel a few different strategies can be used. we will focus mainly on the wing, but principles hold true for any inserted stiffener. First thing is to get your spar cut to length. Don’t find yourself with glue and no spar.
If possible I will generally default to splitting the panel into two parts. Often I will then lay the parts down(top side up), butted together as they will be glued. Stick a couple strips of tape down(the low tack green or blue stuff) to hold the panels together as if they were hinged. Then I pick the parts up and fold it like it is hinged, so that the pieces are back to back. This leaves me with the top sides semi protected, because they are against each other. For Depron I use Gorilla glue, EPP gets Welders glue. Now the area needing glue is right up front, easy to get to, put a bead down. If you don’t want to smear glue around with the tip of the bottle use a popsicle stick to smooth/spread it out. With the parts still folded against each other lay your spar in place. With one hand it might need a little straightening, so have a paper towel or napkin ready to wipe off with. Once the spar is in place, flip the other panel into place. Usually it is good practice to put a couple strips of tape down on the untaped side of the wing. If using Gorilla glue I often put tape over the glued area to keep it from growing out and all over. With Welders glue you don’t have to worry about it foaming up/out, but it can goo out and glue the part to whatever it’s on. For Welders the safest thing is to lay it on wax paper. most of the time I just flip it over every couple minutes, checking for anything oozing.
When gluing into a slot using welders find it good enough to put the tip on the foam, and put a bead into the slot. Not too much, put it in from the top side of the panel. Still working from the top side push the spar through the glue. The goal is to have enough glue to bond, not enough that you push it through the slot. Almost always you will need to clean off some amount of glue from the bottom side of the panel. This works best when the spar width is less than the thickness of the panel. As soon as it’s in and cleaned up lay the panel on a flat surface and be sure any twist is removed before the glue can set.
Gluing into a slot with Gorilla glue is almost the same as with Welders. Only difference is that I will put the glue on the spar. Sound easy, it’s not… Typically the result of trying to run a bead of glue down a long skinny strip, on BOTH SIDES, not pretty. I’ve tried using a bunch of stuff for the next step, nothing works as well. So what I do is pinch the spar, yep on the glue, and spread it into an even layer of glue. Obviously this will leave you needing to clean off your fingers. Put the spar into one side of the of the slot, as you lower the spar into the slot spread the slot open enough to keep from scraping the glue off the spar. As mentioned before, when using Gorilla glue I like to put a strip of low tack tape over the spar, both sides of the panel.
Now onto low temp hot gluing, reserved for spar and panel having the same cross section. Using hot glue to bond these parts can keep your fingers much cleaner. I like to get the spar into the panel, centered ready to go, laying on a flat surface. Spread the panel apart enough to get a dot of glue onto the spar, start at the one side of the spar, and only glue one surface of the spar to allow you to keep the slot spread a little. Second dot will be the opposite end of the spar. Third dot goes right in the center of the spar, as you may have guessed we are working the rule of halves. Fourth and Fifth will split the first few dots placements in half. Do the same thing for the other surface of the spar. It is likely that you will not be able to spread part enough to get a dot in every time. You just have to put the glue gun tip on the spar and panel and squirt a little glue into the gap.Hold things together to set the glue as you go. By now you should have a fairly secure couple of parts. Next thing is to put the gun right down on the spar. Angle the tip so as to push the glue out in front, so it will spread out and cover the top of the spar. No need to make the glue come out of the slot, just a small amount of glue will do. Think of it as sealing the spar in place. Flip and do the other side next, then you’re done.