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Nov 08, 2004, 07:42 PM
Valid8r
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Balsa - Dope vs. Polycrylic as a sealer/primer


I'm building my first balsa plane (MM SmoothE) since I was a kid. I am going to paint the balsa components (fuse and tail feathers) as I am not yet up to covering them. I am sealing the balsa with thinned Dope, then I'll sand, and possibly repeat if necessary, then paint.

My question is simply this. I've read in the forums here that a lot of people use Polycrylic (water based). So, what's the difference advantages/disadvantages to both? I chose to go back to my roots with the Dope when I accidentally warped my elevator by using a water based paint primer.

Comments? Thoughts? Advice?

I'd like to get rid of the fumes (though they do remind me of my early days building balsa gliders and covering with silkspan).

Thanks,
Jon
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Nov 08, 2004, 10:42 PM
Dismembered Member...
arx_n_sparx's Avatar
Quote:
Originally Posted by Valid8r
I chose to go back to my roots with the Dope when I accidentally warped my elevator by using a water based paint primer.

Comments? Thoughts? Advice?

I'd like to get rid of the fumes (though they do remind me of my early days building balsa gliders and covering with silkspan).

Thanks,
Jon
Jon: That sentence kinda says it all. Wood warps when you apply anything water based to it. I use WBPU on my foamies.....I've heard of guys using hair spray to seal balsa, but I don't know the specifics....

Brad
Nov 08, 2004, 11:28 PM
MTT
MTT
I care about rising air !
MTT's Avatar
I have had good results with dope and WBPU ( water-based polyurethane )
The dope will seal and harden the balsa, and the polyurthane gives it a nice finish.

You can even alter the appearnce of the balsa, by mixing wood stain into the dope, I have done this on my Krick Grunau Baby scale sailplane :

Before :


After :


Michael
Nov 09, 2004, 04:20 AM
Valid8r
Valid8r's Avatar
MTT - Nice. One question. Can WBPU be used directly on balsa without first having sealed with Dope?
Nov 09, 2004, 07:31 AM
MTT
MTT
I care about rising air !
MTT's Avatar
Quote:
Can WBPU be used directly on balsa without first having sealed with Dope?
Not sure...
Never tried it.
But the WBPU is very thin, and "water-based" so I would think that the balsa would soak up a lot of it.
If you seal the balsa before applying the WBPU, with dope or sanding sealer, you will have a nice, smooth surface, which will require only a thin coat of the WBPU to achieve a nice finish.

Michael
Nov 09, 2004, 07:34 AM
Valid8r
Valid8r's Avatar
Makes sense - I'll give it a try. It's funny though, I don't hear/see/read that much on the use of dope on balsa in these forums, it's almost all WBPU so I was beginning to think that dope was out of the picture these days and replaced with WBPU...

Jon
Nov 09, 2004, 08:08 AM
Registered User
rcav8r2's Avatar
You may want to cover the bare wood with some .5 ounce cloth. It is VERY easy to do using the BBPU. Tons of threads here. Only use on sheeted surfaces, Doesn't work too well on open surfaces.

The reason I mention this is strenght. When left un-covered, the wood tends to split very easily along the grain after just a little bit of use. A few years back, I just painted the fuse of a small speed 400 glider. Looked great, untili I flew it a few times. Tiny cracks formed all over the place. And this was w/ no crashes. Well I ended up sanding most of the paint off, and covering w/ .5 ounce fiberglass cloth and WBPU. No warping, even on the small control surfaces, but now the fuse is almost as strong as a ball bat. Weight gain..... less than .5 ounce for the fuse. This includes a few coats of paint, and a final layer of WBPU to protect the paint.
Feb 03, 2008, 04:53 AM
Registered User
I use 50/50 sanding sealer and thinner in two coats. Sand between coats and don't apply either coats heavily. Then wax down with auto finish 'cut down' treatment. Thoroughly polish the wax off untill it serves only to provide moisture seal and help keep the balsa from splitting. You can add dry powder tint (for shaded color) to the 50/50 if you want. If the dry powder tint is to gritty just dampen a little and use a metal roller to grind it on a sheet of glass. Above all keep the weight down!
Feb 03, 2008, 10:24 AM
Zor
Zor
Suspended Account
Quote:
Originally Posted by Valid8r
I'm building my first balsa plane (MM SmoothE) since I was a kid. I am going to paint the balsa components (fuse and tail feathers) as I am not yet up to covering them. I am sealing the balsa with thinned Dope, then I'll sand, and possibly repeat if necessary, then paint.

My question is simply this. I've read in the forums here that a lot of people use Polycrylic (water based). So, what's the difference advantages/disadvantages to both? I chose to go back to my roots with the Dope when I accidentally warped my elevator by using a water based paint primer.

Comments? Thoughts? Advice?

I'd like to get rid of the fumes (though they do remind me of my early days building balsa gliders and covering with silkspan).

Thanks,
Jon
Hi Valid8r

Comments? Thoughts? Advice?
You pretty well got all you need by now but throwing in my grain of salt.

You start with a well smoothed structure finished sanding with 400 grit or so.

Dilute 50/50 nitrate dope (NOT butyrate) and apply with a soft brush (3/4" or 1"). VERY lightly sand with 400 grit just to remove any "pimples" that may exist.

Apply very light fabric no heavier than 3/4 Oz using a second coat of thinned dope. Use a stiffer brush to help get the dope through the fabric. Sand again as before for the same reason.

Apply a third coat (now butyrate dope) diluted 75% dope 25% dope thinner. You should have by now filled the pores of the fabric. Retouch that third coat where the pores are not fully filled. Sand again lightly 400 or 600 grit.

Apply your color skeem; butyrate dope or polycrilic.

You are done.

Good luck with a nice light weight finish for a light weight model.

Zor
Feb 03, 2008, 11:45 AM
B for Bruce
BMatthews's Avatar
First off I've found that WBPU doesn't sand worth a darn. It is also much heavier than dope. Mind you it is heavier because it leaves behind more solids. But this means that WBPU is a terrible sealer for a framework on two counts. First it doesn't sand well and second it ends up heavier. I'd say stick to dope if your later plans are in tune with sealing the wood at all.

What you seal the wood with, if anything, depends on what you want to use as a covering material. If you're going to use a tissue or silkspan or a plain fabric that you dope on like the old days then use dope. The ability to stick the covering down in this case relies on the thinner soaking through the covering and softening the dope on the wood for its adhesion. WBPU will not soften in the same manner and in fact would act as a release agent most likely and make it near impossible to get the covering to stick.

If you are going to use a heat activated adhesive covering like the plastics or iron applied fabrics then DO NOT SEAL THE WOOD with anything at all. Nothing, nada, zip.... Anything on the wood in that case other than PERHAPS one of the suggested and compatible heat sensitive aids like Balsaloc in specific areas will hinder or prevent the covering adhering correctly.

Finally my own experience with WBPU and Minwax Polycrylic is that Flecto brand WBPU is great and quite fuel proof. Minwax Polycrylic is some other formulation and it is not fuel proof in the slightest. In fact it turns into a very sticky and gooey mess at the first sign of glow fuel. I also found that it stays more rubbery than the Flecto WBPU that is the only other one I've used and if I was using it to bond down a layer of very thin glass fiber I would use the Flecto as it would result in a more stable and hard base for further painting and be less prone to pressure dimples from handling. I know the electric guys are using it with great success but that's only because the spilled electrons don't affect it...
Feb 03, 2008, 02:18 PM
Zor
Zor
Suspended Account
Quote:
Originally Posted by BMatthews
First off I've found that WBPU doesn't sand worth a darn. It is also much heavier than dope.
>
>
> I know the electric guys are using it with great success but that's only because the spilled electrons don't affect it...
Well said and explained.
I also appreciate your good sense of humor.

Zor
Oct 24, 2009, 09:32 AM
Larry
harlequin's Avatar
Is there a dope equivalent available in the Local Hardware Store, rather than the Local Hobby Shop? My LHS is not so local anymore. Is there a preferred dope for filling balsa, as a base for paint?

Larry
Oct 24, 2009, 11:06 AM
pd1
pd1
Registered User
Quote:
Originally Posted by harlequin
Is there a dope equivalent available in the Local Hardware Store, rather than the Local Hobby Shop? My LHS is not so local anymore. Is there a preferred dope for filling balsa, as a base for paint?

Larry
I haven't found it in hardware stores for over 45 years.
I get the 5 gallon cans of clear from here.

http://www.airsuppliers.com/shop/search.asp
Type in "Randolph clear " in the search window.
210-GL RANDOLPH CLEAR NITRATE DOPE $28.77QT
210-QT RANDOLPH CLEAR NITRATE DOPE $9.57 QT
701 RANDOLPH ALUMINUM PASTE (Mix ratio: one pound of paste to five gallons clear dope before thinning) $13.77LB
9701-5G RANDOLPH CLEAR BUTYRATE DOPE $146.37 5 GAL
9701-GL RANDOLPH CLEAR BUTYRATE DOPE $30.38 GL
9701-QT RANDOLPH CLEAR BUTYRATE DOPE $10.17 QT
A1690-GL RANDOLPH CLEAR NON-TAUTENING BUTYRATE $32.55 GL
A1690-QT RANDOLPH CLEAR NON-TAUTENING BUTYRATE $10.88 QT
E4964-GL RANDOLPH CLEAR NON-TAUTENING NITRATE $31.05 GL
4964-QT RANDOLPH CLEAR NON-TAUTENING NITRATE $10.39 QT

Here's another good place

http://www.aircraftspruce.com/search/search.php
type in "Randolph clear" in the search window

Smaller quantities it's worth the trip to New Hampshire he stocks Sig and Brodack.

http://www.rcbuyers.com/
Last edited by pd1; Oct 24, 2009 at 11:12 AM.


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