I am building a Great Planes RV-4 and am at the point that I need to finish the seam made by joining the two cowl halves. I have some microballoons coming but was wondering what to mix them with to make a putty-like filler to finish that seam? It seems epoxy would be pretty gummy to sand properly, TiteBond might be OK, don't think CA would work. canopy glue maybe. What would be best to make the filler and what proportions of balloons to adhesive do I use to make a workable putty? Or can someone recomment something better for the job than microballoons?
Max

What about fiber glass resin or polyurethane?

A tube of the red bondo spot filler should be in your shop just for things like this. Also used for filling in the little holes in fiberglass. Sands smooth and easy. You know how much I like easy. Not to mention hate sanding!!

To answer your original question, the microballoons would be mixed with epoxy. The more microballoons, the easier the result is to sand. Gumming up the sandpaper shouldn't be a big issue. Just make sure the mixture still has a glossy sheen to it when you're done mixing. If you get the mixture too dry, it doesn't adhere as well. The reddish-brown micro balloons (phenolic) would be better in this application than the white ones (glass spheres).

Yep, Bondo Spot and Glazing Putty is just the ticket for cowl seams. Dries fast, sands very easy and gives the best feathered edges there are. That's what it's made for.

But as for microballoons, I use the stuff all the time mixed with 5 minute epoxy. How easily it sands depends on your epoxy/microballoon ratio. Mix the epoxy first, then add microballons until the mixture is a nice fluffy white and it's as thick as toothpaste or thicker. It'll sand like a dream. You need to work fast though or use slower epoxy.

Dave

Micro Balloons and Z-Poxy Finishing Resin. Dbacque has the formula that dictates sandability.

If the cowling is ABS.. Squardron putty works wonders.. its available in white or green... it makes for a seamless seam.

If you use epoxy + microballoons mix the epoxy first and add the balloons mixing it till it has the cosistency of peanut butter.


It will sand easily and cleanly.

Talk about brain fade, I used to keep and use the spot putty all the time back when I was doing auto body work. I shall get a tube of it and keep for things like this, but I do believe I will use epoxy and balloons here because I am not sure of the integrity of this joint. That cowl fits together like socks on a rooster and I am not sure exactly how much contact area there was for the CA to work. You can't see it because the lower half of the cowl has a flange running all the way around it and the top half is supposed to fit just outside that and you can't see how much is actually touching, so I think expxy and microballoons might actually hold the whole thing together better than spot putty. However, that might not be the case, so I will get a tube of putty tomorrow and have it just in case. Good call guys.

Nothing better in the hobby world!

Kurt

Is "Squadron" a brand name or what? Where do you get it? Is it a brand of spot putty?

It is a brand, and is sold at most hobby shops that cater to the plastic model builder. They sell two versions:

Well, the closest hobby shop to me is Branson, Mo. which is 90+ miles and the closest one that I would expect to have that is in Springfield, Mo. which is 125 miles. I have an auto body shop supply store about two miles from me. I guess I will go there and get some auto body spot putty.

If like finishing resin w/ micro-balloons. Not as gummy as regular epoxy. Sands very easy.

Bondo spot putty and the epoxy/micro balloons will not work well on ABS. The squadron puddy will work best. The best thing you could do is use the ABS parts as a mold and fabricate some epoxy fiberglass parts.

Listen to speed if you want a perfect glass cowl. Finishing resin is good stuff too, it flexes very well, just takes about 24 hours to set up well enough to sand. On ABS I also like to apply some glass cloth to the inside seams. The ABS cowl makes a great plug. You can use a releasing agent, wax or even a balloon over it to use it as a plug.
Go skin that cat Max.

I like finishing resin mixed with microballoons best as it is much easier to sand than regular epoxy glue as noted by several others above. For the seems on molded plastic parts I would use regular two part bondo for any large imperfections and spot putty for the smaller ones. Spot putty will shrink over time and is not really suited for filling large areas in my experience.

Good advice on this thread, but don't forget common acrylic dry wall putty. I like the red bondo too, but I usually end up using either Microfill (when I can get it) or the drywall putty. Once it cures and you have it fine sanded to shape, hit it with CA and it is as hard as rock.

If your not sure of the joint epoxy

Does anyone know how the bondo spot putty, squadron putty, and epoxy/microballoons all compare in weight? Also, isn't there a lightweight 'filler' made specifically for gaps and seams on airplanes? I was told about it but forget what it's called.

i used epoxy/mirco on my mold seems . being that your simply filling a joint, you CONTROL the mixture of epoxy/micro balloons. mix it up to a thick toothpaste like consistancy there is no prob sanding it when you added enough micro balloons. sands easy. ** my molds are fiberglass not abs.


hobby lite is the filler. water based so you can thicken or thin it.

Hi Joe, the weight is all about the same and most of what you are using is sanded off anyway. There are many products on the market sold as a wood filler. As a kid it was called liquid balsa. Today just balsa filler. Light weight spackling is about the best thing I have found in the last decade or two. The tub it comes in weighs more then the product. Sands so easy and when your finished sanding you can add some thin CA and it is hard as stone. Fill a gap then harden and it bonds with the wood. Found in any hardware store. If you pick up the tub and if feels heavy you got the wrong stuff, the tub should feel empty.

Thanks guys.
I guess when I think about a CA hardening coat, I imagine a hard, shiny finish that is strong but does not blend well (aesthetically) with airplane. I am about to start my first composite plane assembly and am thinking about upcoming gaps to fill or repairs to make. The microballoons mixture seems to more closely resemble the material of the airplane, so in my mind it would be less noticeable when painting over. I also have to work against a weight limit requirement, and am starting off behind the 8-ball with an older design, so anything that gets covered with less coats of paint is a benefit.

If the cowl halves are simply butt joined to each other, they will be a potential failure point regardless of
what you join them with. Consider adding a 1/2 or 3/4 inch fiber glass tape inside for a better joint.
As for filler, the auto putty is good,; micro ballons mixed with polyester resin is second when it comes
to ease of sanding and finish. Luck!