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Post  bsadonkill Sat Jul 29, 2023 4:03 am

I know somewhere at some time there has been a discussion on fuel proofing. I am showing pictures of an FW-190 from a die cut Guillow's kit. SIG manufacturing shows pints of clear butyrate dope is available, but not the thinner. Is there a good substitute for dope thinner? I have heard of people using polyurethane as an alternative to dope. I have been told that clear lacquer will also work as a fuel proofer. Eze Dope claims fuel proofing up to 20% nitromethane, but I am using 24% Glowplugboy Rocket 24. I have also heard of using superglue as a firewall coating. I have made up some test squares for testing different coatings. I am Lenning toward the polyurethane, but I might use Eze Dope to seal the pores first. Any input on the subject is welcome. Fuel Proofing  119
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Post  Ken Cook Sat Jul 29, 2023 5:11 am

Clear lacquer will not work for fuel proofing. In addition to that, polyurethane is much heavier for a model that your presenting. If you already doped on the silkspan or tissue and you poly over it, it can cause other issues as well on open bays. Dope is special due to it's flexibility whereas poly isn't the same. Once exposed to the sun, the clock starts and poly over tissue can make your covering like a potato chip.In addition, no repairs can be made via dope because dope and poly are incompatible. One is high solvent and the other is oil based. Poly can go over dope, not the other way around. Polyurethane will work it's just that you need to go super easy with it and not hose it on. It will protect against higher nitro but not for very long. The finish will start to crinkle where it has penetrated.

Sig Lite Coat yields the highest fuel proofing qualities over any dope available. However, dope isn't and never was fuel proof. Generally , most butyrate thinners are compatible with each other. I alternate Randolph butyrate thinner with Sig regulary and I 've done the same with Brodak. The issue is how your applying it and the temperature. Sig thinner was very special as it was the slowest drying thinner available commercially for our use. It enables it to be brushed superior to the others because it has more open time. Brodak is a medium rate thinner and it generally requires a splash of retarder. This is because as a faster drying thinner over Sig, it traps moisture on the surface . I've done some very nice brush finishes with Brodak . It takes a bit of a learning curve to do so though. Sig offered the best finish even for the beginner.

In general, most dopes are good to 10% nitro. So the bottom line is don't spill it or get it onto your dope. This can be prevented a few ways. Build up many many coats around the engine area and wax it prior to seeing fuel. Wipe it off immediately if spilled. High nitro is like pouring thinner on your model and on a 1/2A , high nitro fuels will destroy it.

I have no experience with EZE dopes. I believe it's not a dope but some sort of water based product which claims to be aliphatic. Most of these products are thinned down PVA glues which utilize a alcohol base as a carrier to speed drying and improve flowability. I have never used it but I've been aware of it for years and I hear good results with it. I've just never seen a finish done with it.


I've done many models like your building currently. Because of weight concerns, I use colored tissue for trim followed by Sig Lite Coat. I also use another product with these models called Dave Brown Flex-All which is a plasticizer for butyrate dopes. This keeps the covering as pliable as Monokote and makes it rubbery and puncture resistant. It also prevents the dope from shrinking and crushing the model like the stringers.

I'm assuming that what I see in the pic is a Pee Wee .020? You can run it on 10% which is how I generally use mine for free flight on the dropper. Yes, it is down on power but it offers me a little more time on the ground to set it and it doesn't go out of sight in 10 seconds. I'm limited to space so one thing I do is turn the prop around and lean it up a fair amount on the 10% which offers a nice slow speed but it does the job.

Your model looks terrific , I'm looking forward to seeing it complete and I wish you success. Nicely done. Ken
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Post  Levent Suberk Sat Jul 29, 2023 5:21 am

Suggesting this dope and thinner:
https://www.aircraftspruce.com/catalog/cspages/randbutyclear.php?clickkey=16252

Polyurethane is also works well.

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Post  Levent Suberk Sat Jul 29, 2023 5:23 am

Eze Dope is fuel proof against raw fuel, however hot exhaust gases can dissolve it, didn't try yet.
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Post  akjgardner Sat Jul 29, 2023 7:43 am

I run my engines on a table I made as a senior woodworking project in high school. I re finished it about 5 years ago and used polyurethane. So far it hasn’t been affected at all, not saying it won’t at some time. I run anywhere from 15% nitro to 35%
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Post  bsadonkill Sat Jul 29, 2023 9:15 am

Thanks for the input Ken, akgarder , and Levent. Ken Cook brought up a good point. Weight on this type of model is critical. The fact that it's an old die cut kit makes it a bit on the heavy side. I did a lot of sanding on the back side of the sheets. Even with the sanding, it's still a few grams heavier than my BF 109. It sounds like a good ideal to stay with the Super Coat. It's the stuff I have used in the past. I think I will try the Eze Dope on my test squares and see if it holds up to engine splatter.
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Post  aspeed Sat Jul 29, 2023 11:11 am

I found the only really fuel proof paints are two parts, and they are a bit heavy. Automotive two part clear can be sprayed on, or 30+ minute epoxy that is heated up a bit to flow better, or thinned. I at least use it near the motor. I have not bought any clear automotive paint because it is in an expensive gallon can, but have used two part automotive paint leftovers from various car projects. I think butyrate dope is still a good option for most of the plane though for it's weight and tightening properties.
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Post  andrew Sat Jul 29, 2023 12:22 pm

aspeed wrote:...............or 30+ minute epoxy that is heated up a bit to flow better, or thinned. I at least use it near the motor. ................... I think butyrate dope is still a good option for most of the plane though for it's weight and tightening properties.

I agree. I apply the 30-minute epoxy, then warm it with a heat gun (preferably covering gun since it is a bit cooler). The epoxy will run into every crevice and soak into the firewall and surrounding wood. It's surprising how much the wood can absorb -- if it appears dry after setting up, I may apply a second coat after prepping the first.

I still like butyrate dope -- both tautening and non-tautening Randolph clear available from Aircraft Spruce or other homebuilt suppliers. If you'll use colored covering, the overall weight is minimal.
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Post  cstatman Wed Aug 02, 2023 12:10 pm

as Levant mentioned.. Randolph Butyl from Aircraft Spruce.

its what I use on my FF birds.
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Post  HalfaDave Thu Aug 03, 2023 8:04 am

Hi All,
I agree with Andrew, 30+min epoxy is best. (5min epoxy is soft, mostly filler/hardener)
I warm up the area I am coating, then brush it on.
Soaks up like a sponge. Re coat with same brush/batch, done.
My thinking is the epoxy gets into the pores of wood before fuel does... Smile
I have had good luck with CA too...

Here is a 'fuel proof' story:
My friend Frank loves building. Perfectionist.
Big on fuel proofing. Tank compartments, hatches, firewalls, all beautifly done...
His .40 powered 'Hurricane Harold' had a 12oz tank. Flew great for +2yrs...
Then, he got an electric fuel pump...
Could not understand why the tank did not fill...
Someone noticed, fuel coming out of the tail of 'Harold'.
The fill line to the tank came off, fuselage got filled with 3/4gal of fuel !
Wing and tail got morphed into other planes.
Here is the fun part... Smile
Frank kept the fuel soaked fuselage, the fuel soaked balsa turned into mush...
The 'fuel proofed' nose,
Became a very cool hanging 'sculpture' over his bench... Smile
True story, hope you like,
Take care,
Have fun,
Dave

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Post  rsv1cox Thu Aug 03, 2023 8:23 am

Good story Dave. There are a lot of us "Franks" out here. Why I once...............................
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Post  bsadonkill Wed Aug 09, 2023 7:51 pm

Was doing spit and sputter test with rich running Babe Bee. I used samples coated with both Eze Dope and Butyrate Dope. I would run the engine for around 1 1/2 to 2 minutes. Then I would use my finger to smear it around the square. After a few minutes I would wipe it down with formula 409. Both finishes held up peaty well. One thing for sure u need at lest 3 coats of 50/50 of ether one to seal well. My test stand had a coating of super glue on part of it. It works ok for some amount of sputter, but raw fuel will soften it. Notice the horseshoe pattern left by the engine backing plate.Fuel Proofing  120
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Post  aspeed Wed Aug 09, 2023 8:45 pm

I normally test paints with fuel or straight nitro in a rag that is a different colour than the paint, and wipe it pretty hard. I guess it is not a "hot fuel proof" test. I used to always have polyester resin in the shop for my street rods and it works very well for sealing firewalls and the insides of fuselages. It is also good for coating balsa with a few coats, sanded in between. I put a narrow strip of 3/4 oz fiberglass cloth on the joints too. I'm not real fussy about finishes like some folks, but like things strong and maintenance free. It can be a nice finish though. I used Hobbypoxy or K&B Superpoxy paint back in the day.
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Post  706jim Wed Aug 09, 2023 9:47 pm

Never mind fuel proofing. I doubt that you you will ever get enough thrust past that enormous cowling with any reasonably sized propeller for that engine. I ran into this with a Guillows Thomas Morris Scout kit way back in the 1970's. It would fly with a Golden Bee but the Pee Wee 0.020 mostly just blasted the firewall with little net thrust.
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Post  bsadonkill Wed Aug 09, 2023 11:24 pm

Well, you might be right 706jim, but I going to find out.
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Post  HalfaDave Fri Aug 11, 2023 11:38 am

Hi bsadonkill.
I think the key to fuel proofing is...
Not to pour high nitro fuel on it ! Smile
Seal the wood, so castor goo does not get in. Go fly.
Anything aft end of a Cox Engine is cool... Smile
Raw fuel spillage,
Is the problem,
Your results will vary....
Take care,
Have fun,
Dave
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Post  bsadonkill Fri Sep 15, 2023 10:35 pm

Thought l would do an update. Flying with a 4 1/2 diameter 2P prop on backward. Model had enough thrust to go Straight up and stalled then recovered, but second time around it did not have enough height and hit the ground. As you can see it broke a prop. I didn't have the cowling on because I have not decided how I was going to do the plug wires and the needle valve extension. At any rate it definitely has enough thrust to fly. Need to make shims for down thrust.Fuel Proofing  219
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Post  getback Sat Sep 16, 2023 7:07 am

Thumbs Up Cool deal you got plenty of thrust ! At least just the prop got broken , how much over hang does the prop have from the firewall ? Thanks for the report , hope you can get her trimmed out and alot of flights in . Flying
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Post  bsadonkill Wed Sep 20, 2023 5:31 am

I never answered getbacks question, a 4-1/2-inch prop over hangs about 3/4 of an inch.  That's 3/4 on each side.
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