Sealing balsa wood - RC Groups
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Sep 03, 2012, 06:34 AM
Is my CG correct?
discostu956's Avatar

Sealing balsa wood

Just sprayed some balsa and found some dramas with it.

I had primer filled it, and sanded as much off as possible. This left small patches of areas covered, and other areas of balsa exposed.

The balsa exposed areas soaked up a lot of paint and swelled a lot, and this really showed up the sanding scratches that were barely noticable before. I thought after a coat or 2 of paint it would seal up but it didn't seem to. I've sanded the swelled areas but its not turning out real good.

Just wondering what is the best way of sealing before painting next time? I don't think we can get dope here anymore, and this was the way of old from what I can gather.
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Sep 03, 2012, 07:11 AM
Good Better Best quest.
olmod's Avatar
eurethane (not water based) and thinned with spraying enamel thinner works, light sand between coats till the finish you need.
Sep 03, 2012, 07:27 AM
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TomCrump's Avatar
To me, the most reliable method to seal balsa for painting, is to use fiberglass cloth and epoxy finishing resin, or a waterborne polycrylic.

I have not had any luck painting raw balsa.
Sep 03, 2012, 07:58 AM
Registered User
It doesn't need to be glass cloth and epoxy, tissue and thinned PVA gue works as well. Though tissue and dope is still favourite .

Sep 03, 2012, 08:01 AM
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TomCrump's Avatar
Yup. My first success was by using tissue, but that was a looooong time ago. LOL
Sep 03, 2012, 08:15 AM
Is my CG correct?
discostu956's Avatar
Cheers guys. For a lot of the applications its pretty lightweight, so the glass cloth would be out. But would the PVA alone work alright? What sort of ratio would you thin?

I havn't seen dope here (the modelling kind) for a long time. I'm not sure if its still available, but it seems like its a bit too hard to find
Sep 03, 2012, 08:38 AM
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TomCrump's Avatar
Glass cloth and tissue weigh very little. It'sa the glue, or resin, that adds the weight.

You'll use more glue, or resin, if you don't use cloth or tissue.
Sep 03, 2012, 08:45 AM
Is my CG correct?
discostu956's Avatar
Might just be me, but it seems like a lot of work to just be able to throw a coat of paint over it. When working with timber I always found that it needed one coat, then a sand, then then it was all good to just throw some more coats on it

I was thinking that the glass would take up a lot more resin, or need filling more, to not have the weave showing through
Sep 03, 2012, 08:49 AM
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TomCrump's Avatar
Originally Posted by discostu956
Might just be me, but it seems like a lot of work to just be able to throw a coat of paint over it.
It's a matter of ambition, and how much work you are willing to do.

Yes, it is work, but the results are superior. If the effort isn't worth it to you, paint the bare wood.
Sep 03, 2012, 09:01 AM
Is my CG correct?
discostu956's Avatar
Its just the soaking up the paint thats causing the issues with just painting it, which was what I was intending to ask with the original question. I might not have been clear enough

I hope I don't sound like I'm ignoring your advice, or asking for advice then saying your wrong. Not my intention, and I am appreciative of the advice given here. A lot of the planes I'm doing are small gliders etc, so can be pretty weight critical. And a lot of the others arn't worth putting in a lot of hours on, as the thing will probably end up smashed eventually.

My thinking is that I can seal it with whatever it needs, just to stop the colour coats soaking in, then hit it with a primer filler and sand 99% of that off and that will fill the grain, and then I can coat with colour without it soaking right in and swelling the balsa
Sep 03, 2012, 09:15 AM
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TomCrump's Avatar
Sorry. I took it to mean that you were looking for a smooth finish.

To stop the topcoat from soaking in, I'd apply a coat of a waterborne poly, such as Minwax Polycrylic. or try an oil based poly, such as this :

Let it dry for a good week, before you apply your color coat.
Sep 03, 2012, 09:17 AM
Is my CG correct?
discostu956's Avatar
How to make a slow build last a week longer

Beauty, thanks for your help.
Sep 03, 2012, 09:21 AM
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TomCrump's Avatar
I've tried to rush the painting over the ploycrylic, only to regret it later.

If the moisture isn't completely evaporated, it can form bubbles under the top coat, when the model spends time in the hot sun.
Sep 03, 2012, 09:03 PM
KE your cub.
Curare's Avatar
Stu, pacific balsa do dope, FYI.
Sep 04, 2012, 12:07 AM
B for Bruce
BMatthews's Avatar
You certainly do not want to use coloured paint to fill the wood. It's incredably heavy compared to other clear options.

If it doesn't need to be glow fuel proof one old fashioned sealer that still works well is good old shellac. It seals the wood well, dries fast, sands SUPER easily without clogging the paper and builds up quickly. You can even mix in some talcum powder to produce a grain filler that is again super easy and non clogging to sand away the excess. It's also not a bad primer for a number of options to go over it. But do test first to ensure that the colour products don't react with it.

Something I did notice is that there was no mention of the size of this model or if it has large portions that are open framed.

On smaller models and those with lots of open framing the option for light glass cloth and epoxy quickly becomes less attractive as the size goes down. The smallest model that I could see sealing and protecting with cloth and resin would be something of at least 4 foot span. Below that size the wing loading is sensitive enough that the model will no doubt be more fun to fly if it's not carrying around the 3 to 4 oz of cloth and resin that would be needed to cover it. But on bigger models it's a wonderful way to go if the tricks to hold the weight down are used.

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