Where/when to use epoxy?

[mikevillena]

An alternative to micro balloons is baking soda. Remember when using CA or CA with fillers is that the patch will be harder than the surrounding plastic. This will require a lot of care when blend sanding otherwise you will end up sanding more of the surrounding plastic than you initially intended to.

[Speedfreak]

wow, thanks Mike, it just gets deeper and deeper. I've got some pretty tall waders though.

[Ace-Garageguy]

These are thick fills done with epoxy / micro. Let's see you do this with super glue / baking soda, or just straight epoxy.

It sands very well because it's relatively soft, but it also adheres to the substrate much better than bondo (but it takes longer to cure).

It also doesn't shrink.

A thin-bodied 2-part polyester filler like Bondo Professional glazing putty is helpful to put the final touches on the surface after the rough sculpture is done in epoxy / micro.

The result, with a simulated bare-metal finish.

Edited May 27, 2014 by Ace-Garageguy

[Art Anderson]

What is the point of the micro balloons? I have heard of people using it with a glue for a filler, but don't really understand the benefit over just using epoxy, thick CA alone or in place of a 2 part filler like bondo alone.

Aaron,

As others have stated, the use of powdered fillers such as Micro-Balloons has the capability of "softening" otherwise very hard glues such as epoxies or even CA glue, which makes them far easier to file, sand, even carve with Xacto knives--to get the shapes and surfaces desired.

I first learned about Micro Balloons back in 1981, when a newly made modeler friend in far Northwest Indiana showed me a complete 1/25 scale model race car body (1920's Frontenac Model T Ford) he was scratchbuilding. By mixing Micro Balloons with Devcon 5-minute epoxy, then spreading that over a "buck" he'd carved from a scrap block of that blue insulation styrofoam used in construction, he was able to form that body shell in perhaps an hour or so. Once cured out, he would file and sand it to shape, add more epoxy/MB mix just as we all use commerciallly made putties, and work it down. After he was satisfied with the results, he simply opened up the grille area and cockpit, took the project outdoors, and poured a coffee cup of acetone into the body shell, which dissolved the styrofoam in a heartbeat so that it could simply be poured out through the grille opening.

I used this technique a few times after that to create my own Indy car and USAC Dirt Championship model car bodies--works great!

In case it's not been really explained, micro balloons come out of the realm of high-tech industry and scientific research. Builders of radio control model aircraft discovered the material nearly 40 years ago, finding that when mixed with epoxy glue it made a very lightweight filler that could be used in smoothing out balsa wood cowlings and other fairings on model aircraft, where low weight is highly sought after. They are minute, tiny glass balloons, as fine as grains of talcum powder (which some of us older model builders can remember mixing with Testors plastic cement back in the late 1950's to make our own fillers before putties became readily available for model building), and yes, they can be an irritant if breathed in or if air currents should happen to stir them up to the point they get in your eyes, so protection is highly advised. Of course, wet sanding will keep that dust down, making it into a white mud that washes down the drain very readily. In addition to the glass microballoons discussed here, phenolic microballoons have also been available, but not as popular with modelers over the years. By comparison, pretty much all putties used in autobody use a filler material to make the product easily spreadable, relatively light in weight and more easily filed and sanded to shape--surprisingly, talc is what has been traditionally used to thicken up these otherwise very liquid compounds, and for the same reasons Micro Balloons come up for discussion in forums such as this one.

Art

[Aaronw]

Thanks for the explanations.

[Speedfreak]

How long should you let straight epoxy cure before sanding?

[Ace-Garageguy]

How long should you let straight epoxy cure before sanding?

It depends entirely on the epoxy, and not all are created equal...or anywhere close to equal.

I use out-of-date MSG 285 resin made for real aircraft, because it can no longer be used legally on planes, but it works fine...exceptionally well...for models (and I get free surplus). MSG 285 takes 24 hours for a full cure, must be mixed on a gram scale, and must also be "post-cured" at elevated temperatures to develop full strength.

I also use West-System 105, which isn't as persnickety about measuring, and cures to sand in about 8 hours. It works VERY well as a base for microballoon fillers. Many 1:1 and RC aircraft guys use it.

Because I'm rarely in a hurry building, and have lots of other projects to work on while one cures, these extended times work for me.

Experiment with the material you choose, make notes as you go, and figure out what works best for you.

Edited May 28, 2014 by Ace-Garageguy

[Speedfreak]

I just used straight hobby shop epoxy to fill a hole (from a drill bit) in some styrene, let it cure for 12hrs,.. when I went to sand it smooth it just crumbled like I was sanding a pencil eraser. I now have CA in the hole.

Edited May 28, 2014 by Speedfreak

[Mike Kucaba]

Well IMHO that is not the way to fill a hole. Start by shaving down a piece of plastic sprue, you can use a pencil sharpener to do this real quick. Get a tight fit and even let some protrude from both sides.Use your CA glue and let that dry. snip off the excess and sand smooth. Epoxy glue is just one of many useful tools to use in modeling, but not necessary the best depending on the task at hand.

[Bernard Kron]

I've discussed epoxy on this forum before because I've been forced into relying on it by the fact that I developed a bad allergy to CA. I've been using 5-minute epoxy for about 3-4 years now and have gradually developed techniques so that I can substitute it for most jobs where I would have used CA or, more and more often, liquid styrene cement or even plain old Testors red tube.

As others have mentioned, epoxy is great for bonding dissimilar materials: plastic to metal, wood to metal, resin to plastic, etc. It's also great for non-styrene substances like resin, metal or wood. Obviously, these are all areas where I would have used CA. The downside of epoxy is that in the early stages of the cure it isn't very sticky. Since the time it takes to "set up" starts at around 3-4 minutes for "5-minute" epoxy it lacks the tackiness properties, at least initially, of something like styrene tube cement, thick CA, or an aggressive liquid styrene solvent for a small joint. That’s where, for small joints and fine detail work, CA was great, especially with a zip kicker. Those were the days...

But I've learned to prepare quite small amounts of 5-minute epoxy, let it set for 2-3 minutes, and then, using a tooth pick, pin or other very small pointed object, apply it much as I would have CA with much the same results. So nowadays I use it for engine detailing, for example.

When I was first using 5-minute epoxy I would occasionally have problems with a weak joint. I'm not entirely sure why. I suspect it was dirt or air bubbles, but it would sheet and give way under stress. Perhaps this was Bill’s experience, too. Nowadays I just follow the rules of any good bonding job: clean surfaces and a well applied coat of adhesive free of any gaps.

I swear by epoxy for final assembly. It's very strong and, as mentioned above, has largely eliminated the need to do repairs when attending model shows, meetings, etc. Again, as mentioned earlier, epoxy can be easily cleaned up using isopropyl alcohol. That means not only is it unbeatable for doing windows or other clear parts, but it’s terrific for final assembly. It won’t mar paint! Just wipe it down with alcohol and it’s gone…

So I’m a big fan of the stuff, even if it took a major melodrama to try it. I don’t use it for styrene chassis building, or any small styrene to styrene joints for that matter. For that I use MEK (methyl-ethyl-ketone), the active ingredient in most solvent type liquid styrene cements. But for larger surfaces of styrene to styrene, for any assembly of surfaces that might have been painted already, and most other jobs, epoxy is my weapon of choice.

I’m excited to try slow epoxy as a filler using micro-balloons. And, of course, Bill’s technique of using fine fiberglass sheet with it is beyond cool. These are on my “skills to learn” list, along with scratch building wire wheels…

Edited May 30, 2014 by Bernard Kron

[Art Anderson]

I just used straight hobby shop epoxy to fill a hole (from a drill bit) in some styrene, let it cure for 12hrs,.. when I went to sand it smooth it just crumbled like I was sanding a pencil eraser. I now have CA in the hole.

I learned, years ago, that there are some epoxies (glues) that will not stand wet-sanding--when their surface is wet with water, in very short order the stuff just crumbled like sand, as you mention. The couple of brands I had that happen with were off-brand stuff bought at one or the other of the "-Marts", so I went back to using DevCon--with which I have never had that problem.

Art

[Speedfreak]

Art, like I said it was my LHS's 'house brand' epoxy. I sanded it dry but still, it was like sanding a pencil eraser, the stuff just crumbled. I'll try well known brand next time. Thanks.

[Ace-Garageguy]

All polymer-based products have a shelf-life, and your poor performance could be due to chemical breakdown of the constituents over time. There's a reason why we have to scrupulously watch the expiration dates on aircraft epoxies.

Inaccurate mixing can also contribute to disappointing performance of epoxies, so watch your mix ratios carefully.

[jwrass]

Ace-Garageguy, Bill, I want to try the Microballoon thing as it sounds interesting, any brand you prefer? any ratios you like? I have been reading many of your posts and you give some great advice. I can tell you do your homework!!!! Thank You and keep them coming.

Respectfully jwrass

[jbwelda]

I too use devcon 5 minute epoxy and as mentioned, watching ratios, it seems very strong and certainly sands well and can be used for making headlight lenses, gauge glass, and similar. you can do that filleting thing with it as well. I usually apply it to both surfaces, then wipe off most of it, let it set up for 3 minutes or so and join the pieces. give almost instant adhesion but still allows some wiggle in final positioning. ive have pretty much made it my adhesive of choice for a lot of reasons. for mixing I save up defective CD blanks and mix on one of those. you can mix 5 or 6 small batches on one disk, and I happen to have once bought a 100 pack of fairly defective CD blanks so I am not going to run out of those anytime soon!

jb

[Ace-Garageguy]

.. I want to try the Microballoon thing as it sounds interesting, any brand you prefer? any ratios you like?

The last batch of Micro I used came from Aircraft Spruce. http://www.aircraftspruce.com/catalog/cmpages/bubbles.php

The $9.95 bag will last a modeler a long LONG time. As mentioned, be careful using it, as it's extreme light weight makes for a new experience in airborne dustiness.

I worked out ratios with West 105, which I mix on the gram scale for consistent results. Misplaced during the move, when I find them I'll put them up.

I too use devcon 5 minute epoxy and as mentioned, watching ratios, it seems very strong and certainly sands well and can be used for making headlight lenses, gauge glass, and similar. you can do that filleting thing with it as well.

You guys have got me sold on at least trying Devcon 5-minute. It would be great to have a 5-minute epoxy that actually worked, so I'm in for a test, anyway. Back in the old days, in a pinch we used a Devcon "F" product to repair VW cases that had thrown rods (as long as the bearing webs and oil passages were OK) so they obviously knew their chemistry back when.