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-   -   Filling Balsa Grain (http://www.oldrocketforum.com/showthread.php?t=2156)

mojo1986 09-11-2007 02:46 PM

Filling Balsa Grain
 
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Hi guys. Just thought I'd throw in my 2 cents worth on this topic, since there seems to be plenty of interest on the subject. I've read in one of the forums about techniques where people use white (or yellow) glue to fill and seal the grain. These glues can be sanded only with difficulty and I have found them more useful for stiffening or reinforcing paper areas on a rocket (such as shrouds) than for actually filling the balsa grain. Also, these products are water-based, and all water-based coatings will tend to warp fins.

I use a product called 'BONDO' glazing and spot putty which is used in the automotive refinish business. Because it is not water-based and is readily thinned with acetone or toluene it is great for model rocketry finishing. It is completely compatible with dope (and, in fact, I thin it with dope to get a more workable product, but more on that later) and so it makes a great surface for your final sanding sealer step, if that is part of your finishing technique.

I should point out that this material is available at your automotive and hardware stores under many different brand names. Just be sure to get one that is NOT water-based (I'm not even sure that water-based ones exist, but if they do, stay clear of them for model rocketry applications). Also, I should point out that this product is not compatible with white or yellow glue, even when dried on your rocket (will not adhere to it), so it's important to keep your glues fillets on your fins small and neat for the best possible finish.

The first thing I do is to dilute the product approximately 50/50 with dope. You can use clear or white, or for that matter any color you wish, but darker colors will be harder to cover with your primer coat. In diluted form it flows more readily into the balsa grain. The application technique I use is to mix it to a uniform consistency and then apply it with a stiff piece of balsa (for this, a short piece of balsa stick about 1/4"x1/4" in cross section works well). It's important to use balsa and not a harder material like a wooden or steel ruler, because these can put digs into the surface of the relatively soft balsa. I apply a cover coat to the fin as quickly as possible (it dries fairly quickly) and then use the stick to scrape the surface of the fin hard (the harder the better...........support the fin while you do this). For curved surfaces such as nose cones I use my finger to work it into the grain. A rubber glove is advisable to avoid contact with the skin, and all of this work should be done in an area with good ventilation (outside if possible). The scraped filler can be returned to the bottle. This technique minimizes the amount of sanding you will need to do in the next step. Because there is a fair amount of volatiles in the diluted product, it shrinks down upon drying, so after the first sanding you will probably see that there are still depressions where the grain is. But fear not, sanding is very quick and easy, so I sometimes use up to three coats with sanding between each.

A word about sanding..............I find the sanding sticks my wife and daughters use to sand their fingernails work great! They have a coarse grit on one side and a somewhat finer grit on the other. Best of all, they are flat and stiff, so you can avoid the wavy surfaces you sometimes get when sanding with sandpaper.

I always finish off with several coats of sanding sealer, sanding with a very fine paper between each. The result is a substrate for your primer or topcoat which can be almost as smooth as glass. While such treatment will add a bit of weight to your rocket, some experts maintain that performance increases from the smooth surface outweigh decreases from the added weight. And the bonus is a rocket that looks great! Incidentally, the most fastidious of modelers will apply a filling and smoothing technique to hide the spirals of the body tube as well.

Finally, a word about the butyrate dope I use to dilute my BONDO. It is still being produced in the US by at least one manufacturer and so is available in hobby shops. It is also available under the tradenames 'Testors' and 'AeroGloss' and appears in auctions from time to time on Ebay. More info in one of the other threads on butyrate dope. As a topcoat for your final finish it is, in my opinion, unsurpassed. It is mostly volatiles, so it gives a thin, light hard finish on your rocket. I used the above-described technique for the nose cone of my Astrobee 1500 (pictured below) which was filled with ruts and crevices in the balsa when I removed it from the kit. When dry, dope can be overcoated with Krylon clear or another topcoat of your choice. But always check for compatibility before you spray as you don't want to ruin the beautiful finish on your masterpiece. Incidentally, while Krylon and other plastic sprays can be applied over butyrate dope, the reverse IS NOT TRUE. Dope will mar and ruin a plastic-based finish.

I would be interested in hearing about anybody else's experience in finishing.

Joe

scigs30 09-11-2007 07:51 PM

I always say if it works keep it. I have tried many techniques, and the one that works best for me is multiple coats of Aerogloss sealer. I have tried other techniques and they do work, but for what ever reason they did not fit my needs.

ScaleNut 09-24-2007 09:56 PM

the wife would put >me< outside if I used this stuff. so I have to stick with the non-stinky alternatives. f-n-f, weldbond , ect...

ghrocketman 09-25-2007 10:44 AM

For me, if the balsa filler of choice is not "stinky" with a large Organic Solvent content, it just is NOT right....I don't like any of that water-based JUNK.
Good Ol' Aero Gloss Sanding Sealer and Balsa Fillercoat were good enuff during the golden era of model rocketry, and they work great now (if you cand find NOS)
The only time I ever use fill-n-finish is when I have some sort of a huge gap that needs to be filled in....If "Bondo" was not so darned heavy, I would use it instead.

moonzero2 09-25-2007 01:39 PM

The best method is the one that works for you!
I use to use Aerogloss, works great. But I sometimes have a hard time finding it.
Now I'm using a product I bought a Lowe's, it's called Deft Lacquer Sanding Sealer. I buy it by the quart. It also comes in a spray can, but I've never tried that. The quart is very economical. It's fast drying, 10 minutes. And it's easy sanding. So in a nut shell it's cheap, fast drying, and easily sandable. You can't get any better than that. ;) You can even buy it by the gallon if you want. Three coats, sanded after each and you can't see any wood grain. :D

tfischer 09-25-2007 02:02 PM

Please don't hate me for asking this...

Other than aesthetics, is there a need to fill the grain? That is, if one was ok with the "painted wood" look, could one simply sand it smooth, paint, and fly, without a noticable increase of wind drag or what not?

Just wondering...

-Tim

scigs30 09-25-2007 02:07 PM

Quote:
Originally Posted by tfischer
Please don't hate me for asking this...

Other than aesthetics, is there a need to fill the grain? That is, if one was ok with the "painted wood" look, could one simply sand it smooth, paint, and fly, without a noticable increase of wind drag or what not?

Just wondering...

-Tim


I agree, but for some reason I like filling grain. When I built rockets in the late 70s, nobody filled the grain. The balsa was sanded smooth and one top coat was added. The fins were sanded smooth again and one more coat of paint. No primer.......The grain did show up close, but the rocket still looked nice because of the good build, paint and decal job.

Solomoriah 09-25-2007 03:34 PM

I don't use sanding sealer, I just prime and sand (and repeat...) but I have to say I prefer the smooth look. I really don't know how much difference it makes performance-wise.

Brian L Raney 09-25-2007 05:36 PM

I guess it depends on what you want from your flying model rocket, tfischer. If you're just flying it, then I guess it might seem silly to do all that work just for you local rocket eating tree (who I hear also has a passion for kites, gliders, and RC aeroplanes). It doesn’t care how good it looks going down. :D What was once a pretty boy ain’t so pretty by the end of summer.

If you're never going to fly her, then all those little details make more sense given the amount of time it will spend sitting on your self for display. No sense rushing a 'Show Queen'. You do you best job for that rocket because you know it will never fly.

On the other hand, I want it all: a rocket that looks equally as good on the shelf as it does in the air. All the rockets I build must fly, but I also want them to look like they came straight out from the flying model rocket catalog. :rolleyes:

Smooth, glass-like fins just make the paint and the decal application easier. Untreated fins make the decals stick out more like a sore thumb, and are harder to apply. Of course, with all the 'Peel and Stick' decals that come from Estes today, it may not matter so much.

Decals are suppose to blend in with the paint and conform to the shape of the model's edges, but 'Peel & Stick' decals sit on top of the paint and have a noticeable clear plastic film. Good waterslide decals blend in with the paint and will not detract from the model’s appearance but enhance it. Normally, I use decal sol and decal set solutions to move the decals around and help blend them into the paint. Decal sol and decal set are useless on vinyl 'Peel & Stick' decals, which will cause them to distort and diffuse on the surface rather than blend in.

But to get good decal adhesion, I need to glass the fins--first--before I paint them. Any other way just doesn't cut it.

I find that using Bondo glazing and spot putty, balsa fillercoat, and sanding sealer are the best methods for me. Pacer's Z-Poxy Finishing Resin also does a marvelous job but takes much longer to cure. And there's nothing wrong with just using auto-primers and sanding until smooth.

I still, yet, need to try the Elmers Fill'n Finish method. They just don't sell that anywhere around here. :(

tfischer 09-25-2007 08:15 PM

Just to clarify, my question wasn't "why are you crazy people spending all that time/work?" but "should I really be doing that, even on my basic "get it built and flying" models?"...

I understand that the filling should be done for the models to look their best. Just wanted to make sure that it wasn't something that shouldn't be done, even if looks were secondary to flying (and time)...

Thanks for the info!
-Tim


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