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Old Dec 14, 2005, 11:49 PM
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Iron On vs Dope Only

Hi, I'm building a 1/2 A plane and was just wondering if its possible just to coat the fully sheeted fuselage with dope only (and no covering). I will still fuel proof the firewall and surrounding area with thinned epoxy (and cover the wing with regular iron on).

Will a dope only treatment work? Read somewhere that it can be used to waterproof balsa, so I guess this would be like varnishing a wooden cabinet?

If its workable, how many coats will I need and should I thin the dope before application?

Also, will a dope only coating be lighter than, say Solar film (about 2.2 oz per sq yard, I think) Any tips and thoughts would be much apprciated.

Thanks.
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Old Dec 15, 2005, 04:10 AM
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Dope only doesn't work well. You would need too many coats and too much sanding to get a decent finish and then it would crack and split almost immediately.

Dope and tissue works well but it's more work than covering with film. You can get it as lighter as standard solarfilm if you do lots of sanding but if youre not careful it can come out quite heavy. I use it for models when I want a paint finish e.g. camouflage or other scale finishes that you can't get with films.

Films are easier.

Steve
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Old Dec 15, 2005, 04:27 AM
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OK, then thanks. How about thinned out epoxy like I use for the areas exposed to fuel?

Just like the look of bare wood.
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Old Dec 15, 2005, 01:33 PM
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Dope only works. Steve, I don't know why you found it doesn't but I did dope only on many, many of the smaller control line and radio control models over the years and it was fine.

However using the dope to hold on a layer of white tissue does add a fair amount of strength that will help avoid the wood splitting during less than ideal landings.

As long as you're using butyrate dope there's no real need to epoxy coat the nose either. Although it WILL help avoid the slow speepage of raw fuel into the wood that seems to happen over time.
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Old Dec 15, 2005, 03:42 PM
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Quote:
Originally Posted by BMatthews
Dope only works. Steve, I don't know why you found it doesn't but I did dope only on many, many of the smaller control line and radio control models over the years and it was fine.
Well there you go efish. You can trust Mr Matthews.

I guess it was either my bad building or my bad luck. When I've tried dope on its own the finish hasn't lasted long. It's always cracked and often bits have flaked off. Perhaps my surfaces were just too flexible or something. After a few attempts I just gave up and added lightweight tissue which fixed all the problems.

Steve
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Old Dec 15, 2005, 04:31 PM
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To add.

It really helps if the first coat or two is thinned to the point that it's about as thick as homogenized milk or even a bit thinner than that. This will let it soak into the grain more and get a firmer grip on the wood. It'll also make the grain fuzz stand up more so you can sand it off lightly with fine paper. You'll need to do that for the first couple of coats. After that you can use thicker dope for a faster buildup. It'll grab the thin stuff easier. By the third or fourth coat you should be seeing some nice shine coming up. The grain will still show unless you use some for of filler beforehand. Classically talcum powder and dope is standard. Apply the thin coats and then the filler and then clear top coats.

If you use thick stuff right off the bat it won't grab the wood well and under the right conditions it can peel off like a snake shedding it's skin. Steve, perhaps that was your problem?
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Old Dec 15, 2005, 08:43 PM
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Quote:
Originally Posted by slipstick
...........or my bad luck. When I've tried dope on its own the finish hasn't lasted long. It's always cracked and often bits have flaked off.
Steve
Like Bruce said, thinning helps. For brushing, I usually mix 1/3 thinner, 2/3 dope; for spraying, a 50/50 mix is a good starting point -- this helps with penetration and will generally give a smoother finish. For solid surfaces, be sure to use a "low shrink" or non-tautening dope -- the high shrink dope will continue to shrink over the life of the dope and can actually bow a flat fuselage side.

Steve, if your dope was high shrink and applied over a very stiff surface, it may have been possible for it to literally pull a crack into the coating. High shrink dope is also really bad about pulling away from 90 degree corners (rudder to fuselage joint) and producing a bridge that can be easily cracked with a finger or other object.

Lastly, if you get a pinhole or have an area that is not completely sealed, oil can migrate along the underlying wood grain and cause the dope to lift.

andrew
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Old Dec 16, 2005, 02:24 AM
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Wow, thanks everyone for the very helpful info and tips! Will give it a go and probably coat put a layer of silk on the fuselage bottom for bad landing protection.

Thanks again!
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Old Dec 16, 2005, 05:24 AM
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i found dope only worked..but only for a time. Any warping of the wood due to slight humidity changes would crack the surface: that's why tissue is recommnded..it has real strength.
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Old Dec 16, 2005, 08:57 AM
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I have always used tissue with dope for covering sheet balsa. The additional cross fibres of the tissue add a lot of strength. This also allows use of a lighter grade of balsa, so there is little or no weight penalty. Doping (or more recently, the use of Krylon clear spray) works, but over time the balsa will split along the grain.

John O'Sullivan
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Old Dec 18, 2005, 10:46 PM
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Thanks. Guess I'll have to give the dope only coating a little more thought.

Then again, I live in Singapore - temperature and humidity here is relatively constant year round.

Will probably just try it on a piece and stress that for a while to see what happens.
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Old Jan 01, 2006, 07:20 PM
MTT
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As already said, dope only is not so good.
But one or 2 coats of dope, and a coat of Polyurethane on top works well, and results in a nice finish.

I did this on my Grunau Baby scale sailplane :
2 coats of dope, with woodstain mixed in, and one coat of waterbased polyurethane.
Once the polyurethane had cured, I "sanded" it with steel wool, to smoothen the surface:


However, I don't know if this is fuelproof.
Obviously, that's not an issue for me....
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