Preferred fill materials for scale details

I've got a 1/3rd scale Iris project that I've been working on (and off) for the past 4 years. Been brushing the dust off of it again recently and decided to spend some time focusing on the bands. According to the scale drawings I've got, the band near the top and the one near the bottom each taper on the top and bottom. I've cut rings from phenolic body tube for the flat portions of these bands, but now I need to fill in above and below, tapering back down from the band to the body tube. So, my question is what kinds of materials people here like to use for something like this.

I've already done some work with Fixit putty, but it is pretty hard to sand after hardening, and I'm a little concerned about reports of how brittle it is. I ran out after filling my BT spirals and doing some fillets and filling on my through-the-wall fins. I was planning on getting some more to just finish the job, but thought I'd query the community here to see if I should get something else for this, instead. What would you guys use? Epoxy putty like Fixit or Milliput? Epoxy with microballoons? Bondo?

On smaller models I use either a paper shroud or wraps of paper to approximate the proper shape, then harden and bondo it, and finally sand to shape. Trying to fill in the difference without some sort of easy to work with support material is probably just going to make you crazy. I'd go for the paper in this case, I think.

So, are you saying to wrap strips of paper of decreasing widths to match the taper I need? If so, what do I use to adhere it? Epoxy?

BigBlueDart said:

So, are you saying to wrap strips of paper of decreasing widths to match the taper I need? If so, what do I use to adhere it? Epoxy?

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Yes; if you're using fiberglass, carbon, or phenolic main tube, then bond the first wrap with something friendly to both paper/cardstock and whatever material you're using. Epoxy works. I'm certain there's some sort of contact adhesive that would work as well. After the first wrap use wood glue, white glue, CA, epoxy, whatever suits your fancy. I prefer wood glue or white glue for wood fiber-wood fiber joints, or CA if I'm in a hurry. If you're flying into the high mach numbers - I'm assuming you're not since you're scale modeling - then you'll probably need a surface material to protect your detail.

White glue to adhere and thin CA to harden the paper, fill then sand to shape.
If its fiberglass use epoxy.
How thick are the tapers? you could use strips of balsa ripped across the grain so it flexes easily, glue then harden with CA and sand to shape.

rharshberger said:

White glue to adhere and thin CA to harden the paper, fill then sand to shape.
If its fiberglass use epoxy.
How thick are the tapers? you could use strips of balsa ripped across the grain so it flexes easily, glue then harden with CA and sand to shape.

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It's tapering down from the thickness of the band (which was cut from an unused section of body tube) over a width of about half an inch. This is on 3.9 ID body tube, by the way. The ripped balsa idea sounds interesting, but I'm not sure I'm understanding exactly how your saying to do it. Sounds kind of tricky, really.

Right now I'm leaning towards layers of paper built up (should only be a few to get the thickness needed). After that would you guys suggest a layer of microballoon laden epoxy to smooth it out?

Thought I should include a picture of what I'm talking about. As you can see, I have a band that is from a sliced off section of body tube that has been cut and stretched to fit around the body tube. This has been epoxied in place. I'll be filling in the gap in the ring with a section from the other piece that will be going between fins after I do that. The taper I'm talking about needs to go from the top of the band to the edge of the body tube, as well as from the bottom of the band down to the line drawn on the tube.

Balsa bends easier 90º to the long grain, if you cut a strip across the grain (it has little structural strength) but it will wrap easily around the tube.

For detailing I use styrene plastic. Hobby stores sell all kinds of styrene strips, tubes, rods, sheets, and other interesting shapes. The thin pieces bend easily, and can be heated with a hot air gun to increase flex. Use CA to glue styrene to paper, plastic cement or solvent-type cement to bond styrene to styrene. You can also add styrene details to primed surfaces using CA.

rharshberger said:

Balsa bends easier 90º to the long grain, if you cut a strip across the grain (it has little structural strength) but it will wrap easily around the tube.

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Thinking on it, this method has merit. I've used this trick on a few occasions without thinking about it. Especially since LOC 4" tubing is .050 thick, a 1/16" piece of balsa would work nicely. Seconded.

Thinking more about the balsa, now that I understand better what you're saying. So, the steps would be adhere, saturate with CA, and then sand/cut to shape? Would I want to finish off with something tougher on the surface?

BigBlueDart said:

Thinking more about the balsa, now that I understand better what you're saying. So, the steps would be adhere, saturate with CA, and then sand/cut to shape? Would I want to finish off with something tougher on the surface?

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CA should be sufficiently hard, but you can use something else if you want to to make it tougher.

I would likely try to do a wipe of filler to get that little taper. Have used this technique at work to get similar shapes.

Mask off the edge of the ring with masking tape and mask off off the tube at the point where the taper ends.

Cut up an old credit card to get a stiff card to wipe the filler with.

Mix up some epoxy and microballoons to a consistency that gives a good wipe without running, sagging or crumbling.

Apply a wad of filler and use the card to spread it as perfectly as possible between the edge of the outer ring and the masking tape. Let the corner of the cut card spreader ride at the edge of the masking tape on the tube to cut the filler to zero at that point. Keep the card square to the tube. Then wipe off the excess filler from the tape on the ring and on the tube. If the tapered wipe looks good, remove the tape.

If the taper ends at the end of your body tube, simply let the wiper ride there and on the edge of the ring.

This will get you 95-99% of the way to your needed taper with no sanding.

I have also used various sorts of small blocks to make wipes like this. The key is to find a way for one end to ride nicely at the tape/tube intersection.

So, in the past I've apparently never used enough microballoons with my epoxy. So, I'm trying it out with just a popsicle stick glued to a piece of cardboard for now. I'm finding that the result leaves a rather porous surface with lots of pits. I sanded it down, but it's still a bit rough. Any thoughts on how to fill the filler? Maybe top it off with an epoxy/microballoon mixture with fewer microballons so it sets smoother?

On my 1/3.7 size IRIS, I crafted those tapers, or transitions, from 65# card stock. Worked great.

Wow! That looks great! I think what I need to do on my next project is see if I can find someone locally that's really good and this scale detail stuff that will do a build with me. It would be good to learn some techniques and get better at all of this.

Part of my problem is likely that I tend to jump in the deep end of things and I attempt new things directly on a rocket I'm building. I'm trying to buck that trend. To wit, I'm testing out different mixtures of epoxy and microballoons. The left side was my first attempt and ended up a bit pitted. My second attempt is on the right side and is looking a lot better.

As you've found, sanding down the epoxy/microballoon mixture (when cured) will leave some pitting. That's normal, but not a problem. Just go back and fill the pores with any decent putty, let cure, and re-sand. Bondo Glazing and Spot putty would be a good choice.

BigBlueDart said:

So, in the past I've apparently never used enough microballoons with my epoxy. So, I'm trying it out with just a popsicle stick glued to a piece of cardboard for now. I'm finding that the result leaves a rather porous surface with lots of pits. I sanded it down, but it's still a bit rough. Any thoughts on how to fill the filler? Maybe top it off with an epoxy/microballoon mixture with fewer microballons so it sets smoother?

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Squadron Putty diluted with Testors Liquid Cement.

PhlAsh said:

Squadron Putty diluted with Testors Liquid Cement.

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Sounds interesting, I may have to try this recipe, the TLC probably extends the drying time too.