Preparing Balsa for Painting?

I've been using West Systems epoxy for about ever and it's arguably the best finishing resin available. The problem with NOT using the lightweight glass cloth is that you have to use a LOT of resin to fill the balsa texture, the texture will probably show anyway over a period of time, plus not using the cloth will allow more of the resin to soak into the balsa resulting in a significantly heavier model. And let's not forget the dozens of hours of required sanding. (Once a properly applied glass cloth application is cured, there is no sanding of the surface.. you just prime. After you finish the application of the cloth with resin, all you then need to do is fill the weave with a lightweight primer. I've never had to use more than one coat and then sand most of it off for a permanently smooth, ready to paint, surface). To apply the cloth, I spray a VERY LIGHT dust coat of 3M77 adhesive over the entire area. I then carefully lay down the cloth making sure there are no wrinkles. To finish up, I then use a plastic playing card as a squeegee to spread the resin over the cloth. This is where you could screw up if you don't pay attention because all you want to do is to make the cloth look NOT dry. If you see wet puddles that glisten in the reflection of light, then that area is too wet. You should be able to see the weave of the cloth, it should look wet but not puddled, but you should not be able to see "white" areas that would indicate that you don't have enough resin on them. Don't use anything faster than a 1 hour cure resin to give you enough time to work with it to prefection. Now you're on your own.

MJC

"Chris W" wrote in message news: snipped-for-privacy@cox.net...

West Systems epoxy resin is probably the best, but comes in large quantities. If you are going to do a lot of glassing, it is the way to go. If you want to try glassing and are not sure you will do it again, or often, you can try Pacer Z Poxy. It comes in a smaller quantity and is available at a lot of LHSs, although it is more expensive by the oz., the total outlay will be a lot less.

Epoxy (regular or resin) sands like a rock and the epoxy itself adds very little strength to the glass. You want to get as much back off as you can before it cures. Using a playing card or spreader made for the purpose works well. Use wadded up toilet paper to remove excess puddles before it sets, then follow with the spreader. If you sand it, use very fine wet and dry. I use nothing more than 600 grit. It will cut through the glass easily, so be careful.

Keep in mind that the object is to keep the weight down. Use as little resin, primer, and paint as possible.

There are about a million little tips for the process. Do a Google search of this group.

JR

Unless you need the strength, glass and epoxy will add unneeded weight. The use of tissue and dope will provide a good, smooth base for almost any finish. For curved surfaces, make the pattern ahead of time, slitting as needed. A small overlap can easily be sanded out later. Use nitrate dope to adhere the tissue to the balsa. Just brush it on unthinned, put down the tissue, and use thinner to soak it through. Add a few coats of thinned dope after about half an hour drying time per coat, let dry overnight and then sand with 400 weight paper.

Finish off using a sprayer with whichever top coating system you choose. The results will be very smooth and even.

Jim - AMA 501383

Water based polyurethane mixed with talc (or commercial sading fillers) works great for prepping balsa. Without the fillers, you will need to apply several coats to actually fill the grain. This stuff also works great with light tissue over the balsa. This will add a little more weight because of the thickness of the tissue and paint.

About 20 years ago there was an article about using resin directly over balsa without any fiberglass cloth. Even lightweight cloth greatly improves the integrity of the wood. I tried resin-on-wood and wouldn't do it again.

In the resin question: Sig markets a Polyester Finishing Resin. I have not used it, however if it is anything like the old K&B polyester finishing resin then its has to be good. While the smell was definitely polyester it was excellent and two coats were easily sanded to a smooth finish over bare balsa. Over light glass cloth was much better. ZAP (Pacer Tech.) markets a great epoxy finishing resin. Used with light cloth, a couple coats will give a light finish base, however in my experience 3 coats does better. If I were ever to do silk/silkspan again, I would first brush on a light coat of epoxy resin. Epoxy resin can be thinned slightly, however brush some on and use your "money-cote" heat gun. It will run gin-thin, soak into the balsa and can be wiped fairly clean, leaving a surface ready for tissue/silkspan/silk, etc. with minimal sanding.

Horrace D. Cain ~~~~~~~~~~~~~

Chris,

Unless you are creating a show model or a competitive scale model, I want to try and talk you out of glass, epoxy, and paint of any kind.

It usually makes an otherwise fine flying model so heavy that it lands like a scud missile.

If you prang it up, you don't just glue the wood and patch the covering. You must do BODY WORK, just like when you prang your car. Who needs THAT? ;-)

If you are building a looker instead of a flier, there's been lots of great advice already given to you.

Good luck,

Bob

Bob, with respect, I must disagree with you on the weight. Some of the lightest models I have built have had glass & epoxy finishes. Done properly, it adds a lot of strength for very little weight, in fact the weight gain can be similar to that for lightweight tissue and dope.

Only if he doesn't pay attention to the sage advice already offered. The weight 'penalty' for thinned epoxy and .6 oz. glass cloth is literally negligible. The difference in weight between a polyester film covering system and a properly-applied epoxy/glass covering system is >gramsmust do BODY WORK, just like when you prang your car. Who needs THAT? ;-)

No such thing as a crash-proof model. Polyester films do not make a model less susceptible to crash damage. OTOH, a properly-applied epoxy/glass covering system will most assuredly make a model _much more_ resistant to hangar rash. A moneycoat model can easily have a wing punctured. An epoxy/cloth model is a very tough bird when it comes to bumps and scrapes.

No covering system can accommodate a Figure 9 Maneuver.

Indeed, some of the worst flying models (and flyers, for that matter) have been the really glamorous jobs seen at Top Gun. In most cases, the builders concentrated on the details so much they forgot about all the weight they were adding. Problem is, those details are not in the covering system itself, they're added to the covering system in the form of a bajillion fake rivets, layers and layers and _layers_ of paint/decals, incredible pilot figures and cockpits, and so on.

Lastly, adding a pound to a ten-pound model doesn't make it a scud missile, but it does raise the wing loading and in a very large percentage of models, actually makes the model handle better, not worse.

If you want to test that theory, go find a set of plans for an original Big Stik. Build two. Cover one with moneycoat. On the second, sheet the wings and cover the whole thing with thinned epoxy and .6 oz. glass cloth.

There is a marked improvement in the handling characteristics of the epoxy/glass version over the film version, and while some of that improvement is due to the little bit of weight increase raising the wing loading, the major improvement comes from the increased stiffness of the wings themselves.

BTDT.

Of course if you do a lousy job of glassing the model, you can 'prove' that epoxy/glass is less desirable.

Cheers, Fred McClellan the dash plumber at mindspring dot com