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Building Tips and technical articles. => Building techniques => Topic started by: Phil Goldberg on June 06, 2017, 02:20:39 PM
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When making fillets with micro balloons and epoxy what mix should I use for easy sanding? Which epoxy, ( I plan on using the slow cure 24 hr. ) Also what thinner do I use on my finger for smoothing etc. ? Will denatured alcohol work, or lacquer thinner?
Thanks for any suggestions......
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For easy sanding, a little bit talc will be useful. Do not forget, the specific weight of the microballoon is far more less than any other component, so use as much in the mix, as possible. (For this reason, use as thin resin, as you can do. Sorrowfully I do not know non-professional name, or availability of a good, thin aerospace resin: Huntsman (CIBA) Araldite LY 5052 and its hardener.
As a working safety engineer I must draw your attention: do not touch resin (mainly the hardener) by bare hands. Some plastic or surgeon gloves must be weared.
As a thinner*, or to wash instruments, acetone is the best, neither alcohol, nor some thinner can work properly.
*when you work with some foam (white, blue or pink) you can work with only methanol. However, after work, far from foams, you can wash the brushes only by acetone...
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Since fillets are not structural, sandability is paramount. For that reason, I use just enough 15 min. epoxy to bind together. I add a lot of microballoons until too thick to mix. Then add a bit of acetone for easy spreading. Epoxy, of any sort, is hard to sand. Use just the minimum amount.
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I spread it with a finger wet with isopropyl and it comes out nice and smooth. I have no problem sanding it with 320. I may start with 220 if I have to knock a bump down, but it's pretty easy for me to sand, even with the hobby store 50/50 epoxy.
I try to buy the re-branded Bob Smith Industries glue. I used to use Loctite 60 minute, but can't find it anymore, so now I use 30. I haven't graduated to the high-end glue, but my expert friend swears by West Systems epoxy from the boat store.
Rusty
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The more micro balloons you add the easier the filet will sand. Laminating epoxy works best and best is to not add alcohol. You want it thick enough so it keeps its shape but if you go too thick it my be more difficult to shape. Practice on something else till you get what yo consider good consistency then go for it on the model.
Good Luck
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When I first heard about this technique years ago, HobbyPoxy Smooth 'N Easy was the product recommended. That's not around any more, so the closest thing is the Finish Cure epoxy by Bob Smith Industries. It's a 20 minute epoxy for laying up fiber glass and is much thinner that regular epoxy. I mix equal amounts in one of those little mixing cups, about enough to measure about 1/4 of the cup when mixed. Then start adding micro balloons a little bit at a time until you get something like cake icing or a bit thicker. This will fill the cup to the brim usually. It's usually enough to do a typical model. When you add the micro balloons, it really slows down the curing time. You have at least an hour to work. I tape off the edge on the wing and apply it with a popsickle stick and work it around and remove excess. Play with the stuff in the cup to see when it starts to get really stiff and then start in with your finger in alcohol and smooth it out. Take your time, you have plenty to get it right. When finished just let it cure over night. I can usually get it to where I don't have to sand them unless there is an edge where it was taped. I usually take the tape up and work that edge down as the last thing when smoothing. The thing I like best about this method is paint seems to stick really well to it. I have NEVER had paint lift off a fillet made like this.
Type at you later,
Dan McEntee
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I have had good success with epoxy/micro balloon fillets. Bob Smith Industries epoxy (30 minute) works fine for me.
To the above good advice I would add: I use blue painters tape to mask both sides of the fillet line, always. Tape goes on the fuselage and the wing to establish the width of the fillet. You can apply, smear, wipe and the tape keeps it off the "non-fillet" areas. If you remove the tape before the epoxy cures/hardens, the hard line at the tape edge will smooth itself substantially.
If you do the fillets in stages (top right, top left, bottom right, bottom left) you don't have to race the epoxy cure and by tilting the plane, gravity can help the epoxy "flow" into a nice fillet shape.
Whether it is model fillets or silicone bathtub caulking, a taped edge helps you get good results. Don't worry about using lots of tape!
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Best bet, for get the epoxy/micro balloons for filets Go with Poly-Fil. A search on here will show how easy it is to work with and if done properly no sanding. D>K
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Good advice above. I use 30-minute epoxy and keep adding micro balloons until I can't get any more to enter the slurry. Unlike as in one post above though, I find then have time to work, before the cohesion of my mixture tends to dislodge it some as I shape it. So I only do one seam at a time - e.g. upper right root. I go slightly past the furthest forward part of the leading edge on the first fillet of each wing. I too use the blue masking tape and apply with just a popsickle stick with the appropriate curve in it. I remove the tape as soon as my fillet application is smooth and sometimes take any raised edge off of it with my finger or a blade edge. I have let the fillet set with the plane rotated along its axis so that the wings are close to 45o from horizontal, with the fillet both at the top and at the bottom. Neither way has shown itself to be the better, because I always have to wrap an appropriately sized dowel in sand paper to get the curvature uniformly concave, even when it looks about right to start. It sands reasonably, with care. Butyrate adheres very well to the cured fillet after the sanding, which is necessary any to remove the surface residue.
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Yes to everything, and if you catch the stuff at the right state of cure you can
Cuyahoga* cut it with a blade or a "cheese grater" rasp.
* Stupid swipe keyboard. Smart phones are going to be the death of me.
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I like West Systems 410 filler over micro balloons. It's super light weight also.
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I like West Systems 410 filler over micro balloons. It's super light weight also.
I second that emotion! ;D
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...Cuyahoga* ...
* Stupid swipe keyboard. Smart phones are going to be the death of me.
Tim, can't you do as I do on my Android, turn off autocorrect and just use suggestions?
And just to stay on topic; if I get completely exasperated with my 'droid LG G4, I'll suspend it in a bucket of epoxy/Microballoons.
Rusty
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Tim, can't you do as I do on my Android, turn off autocorrect and just use suggestions?
Eh, then I'd have to learn stuff. I'm a rather grumpy user of new technology -- I engineer the stuff, I don't use it.
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Id used the finishing epoxy & microballoons on the Mewgull .
Id masked it with the blue tape a mm or two wide of half Dia. of the round model knife handles .
Cleaned the gook of them , pre sanded area to be applied . Wet / Cleaned handle occasionally ,
Rotated it as id dragged it along . Id spooned - scraped a fairly even - correct dose along the fillet
Mix was near saturated with the balloons .
Scrape the odd bit of flash away as you go . First Attempt ,only one or two spots needed attention - a spot of filler , after sanding .
The smaller - tighter fillets at the tail went particularly well , after all the dread and angst contemplateing the proceedure .
Masking it up accurately to a light pencil line meant there were few dags & lumps to clear after .
The Smaller knife Handle had a tapered end , which mayve helped . Contemplateing them for the Typhoon , just now .
A bit of a pain there adjacent the flaps / root fairings . Better to just get on with it , I guess .
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Eh, then I'd have to learn stuff. I'm a rather grumpy user of new technology -- I engineer the stuff, I don't use it.
WHY, these device manufacturers never show the settings as a simple "tree" baffles me. If they did, people would not get so lost in the settings, and understand how to drill down, find what they want to set, and back out. All quickly and pain-free. Then you wouldn't have to remember so many things, rather you only need to know one thing: how to find it.
Rusty the threadjacker
[/endjack]
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In lieu of epoxy, try using Tite-Bond and water with the West Systems micro balloons. I avoid the hassle of epoxy whenever possible.
When I do need a fuel proof fillet, I use epoxy. I find that the micro balloons speed up the cure time to FAR less than the rated time. Thus, use 30 or 60 minute epoxy.