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Old Oct 30, 2003, 12:49 PM
It's the last plane honey
SlowRider's Avatar
Granite Bay, CA USA
Joined Jul 2003
2,133 Posts
Painting Balsa

I've searched this forum for info on painting balsa and found some information that answered some questions but unfortunately have raised others.

I would like to paint the fuselage of my new balsa project and am wondering if after sanding, can I use standard hardware-store wood primer/sealer and acrylic spray paint? The plane is electric so fuel proofing is not required.

In your experience, is one coat of wood primer sufficient? I'm trying to minimize the amount of paint. If I need more than one coat of primer and then several coats of paint I may just choose plastic covering and limit the painting to small parts of the plane.

My LHS has cautioned that using dope or some of the other fuel- proof sanding sealers with acrylic paint may cause a "reaction" with the spray paint (acrylic/latex?) and not dry correctly, or at the least dry very slowly.

I'm a little confused with sanding sealers. Do you sand, then apply the sealer, or apply sealer and sand?

Any feedback or insight would be appreciated.

Thanks,

Andy
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Old Oct 30, 2003, 01:56 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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Thinned dope works great as a sealer and as long as you give it a few days to dry thourougly it's fine with a spray enamels.

Thin the dope so it penetrates well. It should be no thicker than cooking oil and better yet if it's half way between water and cooking oil. Put on two coats and let dry, sand and then let dry for at least 2 days. Spray on a LIGHT coat of primer and sand lightly. Try to sand just until the grey primer is just thin enough to see through but you're not back to the doped wood. Primer is heavy so don't get carried away. Just get a thin but wet coat on the wood and let dry. Lightly spray with your color and your're done. Use just enough that it looks like a wet film otherwise the gloss will not show up. It should look as smooth as a baby's bottom.

As a substitute for the dope white shellac may work well. Shellac is commonly used as a sealer where sap bleeding is an issue so it should be compatible with the primer. Same rules apply though. Give the shellac a couple of days for the alky it uses as a base to dry completley.
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Old Oct 30, 2003, 03:47 PM
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vintage1's Avatar
East Anglia, UK
Joined Sep 2002
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I'd strongly reccomend butting a tisue skin on first before you start to prime.

In my youth I spent many many days sanding sealing, and rubbing down a balsa warship. It looked immaculate sprayed matt grey. Then I sailed it. The balsa got damp, and the surface cracked up.

I was gutted. I was 14 and you get pretty gutted at 14. Actually you get pretty gutted at any age if you invest enough time into something that goes wrong.

So dope some tissue on first. Then you can use dope and talcum, or sanding sealer, or car high build primer, to get a super smooth surface (rubbing down with wetted wet-and-dry, then go in with standard primer, rub down wet and dry, and spray on the final finish.

Its a lot of hard work, especially when there are concave surfaces.

Quick and dirty for me is to apply iron on film to the bare balsa (once sealed you get bubbles), treat that with fine grit and/or an etch primer, and then spray that.
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Old Oct 30, 2003, 04:28 PM
It's the last plane honey
SlowRider's Avatar
Granite Bay, CA USA
Joined Jul 2003
2,133 Posts
BMatthews, Vintage1,

Thanks for the response! I imagine the dope will have instructions, but just-in-case, do I thin with water? If I seal with the dope do I still need a primer? I had assumed they were interchangeable. My understanding (obviously flawed) was that the sealer sealed the wood so that it wouldn't act like a sponge and soak up all the paint. I though dope was basically a sealer that also protected the wood (and paint?) from fuel. I guess I don't understand why you need to prime if you've sealed?

If I want to prevent the wood from cracking I add tissue, or is that if the wood is to be used in water (ie. as a boat)?

Sorry for the confusion guys, this is my first time painting balsa (could ya tell ). My expectation was sand, spray or brush on some sealer (I'm thinking seal and prime are the same thing), and then spray paint.

It sounds like I need to apply 2 coats thinned dope, let dry, sand, let dry 2 days,apply primer, sand primer until wood just barely shows through, apply color.

Thanks,

Andy
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Old Oct 30, 2003, 10:18 PM
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Jim Ryan's Avatar
Cincinnati, OH USA
Joined Oct 2000
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Unless you're happy with a rough finish with lots of wood grain showing, your best bet is to cover the model with either tissue and dope or with fiberglass cloth and finishing epoxy. If you opt for the latter approach, here's an article describing how to do it:

http://home.fuse.net/ryan/glassing.htm

I glass nearly all my models down to Speed 400 size. Any smaller than that, and I'd lean toward tissue instead. Either way, covering the airframe will make for a better finish than just using sanding sealer.

Addendum:

BTW, if you prefer to use tissue or silkspan, I have an article about that too:

http://home.fuse.net/ryan/silkspan.htm

Jim
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Old Oct 31, 2003, 12:29 AM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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Andy, welcome to the wonderful world of volitile solvents. DO NOT USE WATER ! ! ! If you use model airplane dope you need to get dope thinner. The dope is actually much like a lacquer in that it's a plastic base dissolved in a thinner. Some folks have actually been able to use lacquer thinner but it's a crap shoot. If it isn't the right mix of solvents in the lacquer thinner (lacquer thinner is actually a soup of other base solvents) then it'll make a mess in the bottle. So stick with the proper brand dope thinner for now.

Also I totally agree with the others about using tissue. The dope or other paints will continue to shrink over time and as they do the grain of the wood will telegraph through the finish.

If the size supports the use of light glass cloth you can also use the 1/2 ounce finishing cloth with dope to adhere it followed by the primer and color. I've done this in the past with great success. As a bonus the glass cloth gives you a little extra strength. So does the tissue but the glass gives that much more.

But glass cloth is heavy and the fillers needed for it's weave is heavier again. For larger projects it's fine. For somethin less than, say, a 25 powered model I'd stick to the tissue and dope as a base unless it's just a smaller part of the model like just the fuselage or something.
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Old Oct 31, 2003, 02:13 AM
It's the last plane honey
SlowRider's Avatar
Granite Bay, CA USA
Joined Jul 2003
2,133 Posts
BMatthews,

I really appreciate yours and everyone else's recommendations. I'm going to pick up some dope, dope thinner, and tissue at the LHS and practice this weekend on some stock I have lying around. The plane is too small to try glass, otherwise I'd give it a shot, sounds like you can get some great finishes.

Andy
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Old Oct 31, 2003, 03:30 AM
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East Anglia, UK
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Right. As far as the layers that go on top of the tissue are concerned, the theory is that the dope and tissue together for a tight seal.

The other layers get the surface mirror smooth. And prep it for painting.

A few more tips.

First of all get teh dry balsa as smooth as you can, and use lightweight spackle to fill ANY dents. If your model has coarse grained wood, you can even thin the spackle with water and paint it on, then sand ultra smooth.

Next step is a coat of slightly thinned dope, using as advised above dope thinners.

This soaks into the top layer of the balsa and shrinks as it dries, pulling all the little balsa fibers up so you need a very light sand after the first coat.

NOW apply the tissue. Just cut to shape, using small pieces on compound curves, and dope through it to the dope layer beneath..again slightly thinned dope is cool.

Personally I'd add another coat of full strengh dope here afterwards as well to completely seal teh surface, but frankly you have two options.

What you want to arrive at from here on is a thin layer of stuff that is sanded down to a perfect finish before painting. Dope is very hard actually. So I recommend two other routes.

The first is sanding sealer which seems to be something like dope full of wax? anyway its a lot easier to sand than dope. You can layer that on and rub back.

Or you can use car filler-primer. This is something that basically sprays on like paint, dries through a putty like stage to a reasonably hard finish. Its designed to go over car body repairs to fill all the scratches left by the angle grinders they used to repair your auto with

Once you have got a nice smooth surface after many coats of whatever filler primer you used, and hours of patient polishing with the fine grade abrasives, you now switch to whatever is needed to get the paint finish right.

The aim here is to get a dense pigmented layer of even color, so the final paint has a coat behind it that is a constant color. Typically its a white, red or grey primer you might spray on, and rub down very gently to take the roughness out of it. If you rub through, you need a new layer on.

Then its paint time.

Basdically the steps involved have the following functions:

(i) To seal and provide a barrier between the wood grain and the finish. This is the dope-and-tissue. It also provides a harder skin. Epoxy/glass is also very very good, but I am not too good with it. Once the layer is solvent-proof the job is done. In the case of metal preparation, you might use galvanising, and anti-rust primer in this first layer. The whole purpose is to provide a BARRIER betrween the substrate be it metal or balsa, and the outside world.

(ii) The next layer is all about getting rid of physical imperfections. You want to fill the tiny blemishes and build up a layer that can be cut back to perfection. The requirements here are quite different in the materials. In particular we want it light, thick and easy to cut back. Sure it needs to be hard, not soft..but not too hard. It also needs to go on thickly without running too much. I find car body high build filler primer the best, but light it ain't..

(iii) The next layer is all about getting something to lie under the paint that both provides a good key for the paint, and a consistent color to underlie it. This is partcularly important with a gloss finish, beacause it has a lot of 'sticky' in it, and so is heavy, and needs to go on THIN. It can't have too much pigment either, or it loses its gloss. In absolutely concours finishes you would probably prime, then undercoat with as near the gloss color as possible in a matt paint, which provides most of the color, then spray gloss on that to get the sheen. Sometimes you add another protecticve layer on top - a clear lacquer, matt laquer, or whatever - to get the appearance right. Viz metalflake paint, where the finish is fragile and needs further protection.

That's about it. Each layer has a function, and you can combine the function a bit, but always bear in mind what you are trying to achieve.

If you want a real lesson in painting perfection, visit a Rolls Royce bodyshop.

I learnt a lot living over a series of garages doing both rally car prep and spraying. How to do it properly and all the cheapo shortcuts

In particular I remember asking for some bolts and nyloc nuts..'help yourself to as many as you want..we use em once, and never use em again. We strip the car down every week, and throw ALL the bolts away...and use new.'

That's professionalism.

Needless to say my understanding of what I should do, and the patience to actually do it are seldom in harmony. So my models do not necessarily reflect the standards I have laid out for others to follow.

One of the mst useful tips is psychological. Expect it to take weeks, even months, and then you go in with patient steady approach. If you rush it, and then try to make good lack of preparation on an earlier stage, it takes ten times longer, and looks - er I was going to use a non family friendly word there...but you get the drift.

Take it steady. If you get bored. Stop. Leave it a day, a week - whatever - till you feel in the mood for it. Its always good to let the paint harden anyway

Trouble is with this, you will never dare fly it. Get someone else. If the guys that used to bring back those immaculately prepared rally cars had had to scrape the mud off, bash out the dents, and painstakingly re-fill and respray, they would never have had the courage to drive them the way they did.

Mind you I caught the Irish spraymaster skimping over his primer once 'No point in worrying about rust showing through, that paint 'll be off on the first corner anyway, knowing the way HE drives'

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Old Oct 31, 2003, 11:00 AM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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One final thing that no one seems to have mentioned so far...

COLORED PAINT AND PRIMERS ARE HEAVY!!!

If your model is weight sensitive due to smaller size or you're trying to minimise the flight speed then you're better off just using coloured tissue and clear dope and forget the primer and paint.

If you go ahead with all this you may want to tissue and dope the fuselage let it dry as above then weigh it before and after the painting. It may surprise you...

Years ago someone told me that the color finish on their lovely looking CL stunt model added 7 or 8 oz to the overall weight. Granted there's a LOT of model in this case but that's also a lot of weight.
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Old Oct 31, 2003, 10:39 PM
It's the last plane honey
SlowRider's Avatar
Granite Bay, CA USA
Joined Jul 2003
2,133 Posts
Vintage1,

Thankyou for your thorough treatise on painting balsa! I've read many of your posts discussing RC electronics and have found them insightful and always informative; this post is no less, and is of great benefit to me, and I'm sure will be to others learning the intricacies of painting balsa.

BMatthews and Jim, your comments and response are also very much appreciated. I hadn't expected this level of return but I guess I shouldn't be surprised, considering the talent perusing these forums.

Thanks again!

Andy
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