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DTFB Warped after Painting? If it's a control surface, think twice about flying it.

#1
So here is an interesting thing I learned today, which I think is worthy of knowledge sharing.

I finished scratch building a new Tiny Trainer, and then painted the fuselage of my DTFB (Black foam board) a couple nights ago. I had the unfortunate experience of the foam melting from the paint solvents. In part I attribute this issue to having used the all-black DTFB as it takes a lot more paint to cover, and also in part to me getting a little hasty trying to finish painting in a humid environment. Nevertheless, I went and maidened it the next afternoon, and to my delight it flew quite well and needed only a bump or two of trim.

This afternoon I went to fly it again, and when I did my pre-flight check I immediately noticed that the elevator deflection was very limited. I took the wing off and looked it over and what I discovered was the servo was wiggling around during the control inputs. The paper it to which it was glued had actually delaminated. I made the wise (but sad) decision not to fly it, and instead took it home.

This is where things get interesting. I just finished an attempt to salvage the fuselage of my new Tiny Trainer. As it turns out, because I only painted the top side of the elevator and the foam melted/paper shrunk, it warped the elevator and seems to have continued to warp as the paint continue to dry over the next 24 hours. Since the elevator warped, the warped surface had a very large amount of tension on the control surface - to the point where moving even moving it by hand was taut. The warp in the foam acted like a spring, which added resistance in both directions. I believe this in turn caused so much force on the servo, when it moved around it ended up delaminating the paper to which it was glued.

I hope this helps someone out there, I know it has been enlightening for me.
 

Hai-Lee

Old and Bold RC PILOT
#2
You should consider sealing the FB before painting using Minwax or similar and when covering a very dark coloured material you should first coat the surface with a light coloured high opacity primer. When dry the final colour can be applied in far fewer coats.

I used to have similar issues, (to those you described above), when I started using FB but with the information posted on the forum and a little tailoring the procedure to suit the local FB and now the problem never occurs.

Have fun!
 
#4
Yeah, I am thinking of switching to the flite test/waterproof foam. I had a speed build kit for my first Tiny Trainer, and the waterproof foam seemed to be more durable. In my opinion it's probably the best of foam board I have tried -- the standard white being in the middle, and the black seems to be the worst as the paper seems to delaminate a lot easier than any of the others even when unpainted. I bought the black stuff because it was the only thing in stock at the local dollar tree for several weeks.
 

evranch

Well-known member
#5
@Hai-Lee Which Minwax product do you recommend for sealing DTFB? It's hard to get FT foam in Canada so I usually just end up covering the lower surfaces of my planes with outdoor repair tape to keep the grass/snow moisture out.

I do have a HVLP gun that I could use with liquid for larger planes, but sometimes bombs are a lot more convenient than cleaning the gun.
I've been tempted before to spray Tremclad for its water resistance and glorious bright colours (plus I have gallons at hand), but it looks like it would add a lot of weight.
 

mach1 rc

Well-known member
#6
Yeah, I am thinking of switching to the flite test/waterproof foam. I had a speed build kit for my first Tiny Trainer, and the waterproof foam seemed to be more durable. In my opinion it's probably the best of foam board I have tried -- the standard white being in the middle, and the black seems to be the worst as the paper seems to delaminate a lot easier than any of the others even when unpainted. I bought the black stuff because it was the only thing in stock at the local dollar tree for several weeks.
Oh yes the ftfb is a really good type of foam board
 

BATTLEAXE

Well-known member
#7
@Hai-Lee Which Minwax product do you recommend for sealing DTFB? It's hard to get FT foam in Canada so I usually just end up covering the lower surfaces of my planes with outdoor repair tape to keep the grass/snow moisture out.

I do have a HVLP gun that I could use with liquid for larger planes, but sometimes bombs are a lot more convenient than cleaning the gun.
I've been tempted before to spray Tremclad for its water resistance and glorious bright colours (plus I have gallons at hand), but it looks like it would add a lot of weight.
Always good to see a fellow Canuck representing the great wide, and cold north. I have a bit of a solution for ya, check it out...
This should help you out some, there are other tips in the play list section of the channel you might be interested in as well. There are a bunch of flight vids there too, some are good flights, most are good for a chuckle. It would be much appreciated if you subscribed, good luck (y)
 

evranch

Well-known member
#8
Always good to see a fellow Canuck representing the great wide, and cold north. I have a bit of a solution for ya, check it out...
Thanks, looks easier than I thought. I was expecting to have to spray an even coating, next time I'm in the city I'll grab a can and start wiping. Looks easier than the tape and probably cheaper than covering large areas with expensive ORT tape as well. Left a comment and subscribed, you can check out my crappy channel too though I only have a couple vids and they aren't too exciting. Hoping to find time to post some more stuff next year.
 
#9
@Hai-Lee Which Minwax product do you recommend for sealing DTFB? It's hard to get FT foam in Canada so I usually just end up covering the lower surfaces of my planes with outdoor repair tape to keep the grass/snow moisture out.

I do have a HVLP gun that I could use with liquid for larger planes, but sometimes bombs are a lot more convenient than cleaning the gun.
I've been tempted before to spray Tremclad for its water resistance and glorious bright colours (plus I have gallons at hand), but it looks like it would add a lot of weight.
am curious if using an air sprayer would be less prone to cause warping. I have been considering the purchase of a LVLP paint kit. I have a small 5.5 Gal air compressor which only outputs 3.10 CFM @ 40 PSI and 2.20 CFM @ 90 PSI. I found the SPRAYIT SP-33500K kit some time ago for around $80 US which looks like a decent LVLP kit and quite affordable. I am not sure if I can spray the polyurethane on with this or not though.
 
#10
Always good to see a fellow Canuck representing the great wide, and cold north. I have a bit of a solution for ya, check it out...
This should help you out some, there are other tips in the play list section of the channel you might be interested in as well. There are a bunch of flight vids there too, some are good flights, most are good for a chuckle. It would be much appreciated if you subscribed, good luck (y)
I considered minwax before, but when I went to the hardware store I wasn't sure which kind to get, and this video answered that and more. This is a great tutorial video, thanks!
 

BATTLEAXE

Well-known member
#11
Thanks, looks easier than I thought. I was expecting to have to spray an even coating, next time I'm in the city I'll grab a can and start wiping. Looks easier than the tape and probably cheaper than covering large areas with expensive ORT tape as well. Left a comment and subscribed, you can check out my crappy channel too though I only have a couple vids and they aren't too exciting. Hoping to find time to post some more stuff next year.
I will check out your channel for sure, and I will sub to you just cuz you are a home boy lol! Thanks for the consideration
am curious if using an air sprayer would be less prone to cause warping. I have been considering the purchase of a LVLP paint kit. I have a small 5.5 Gal air compressor which only outputs 3.10 CFM @ 40 PSI and 2.20 CFM @ 90 PSI. I found the SPRAYIT SP-33500K kit some time ago for around $80 US which looks like a decent LVLP kit and quite affordable. I am not sure if I can spray the polyurethane on with this or not though.
You could thin it out with windex, some have used alcohol but it tends to dry to quickly. Be sure to try it on some scrap FB first
I considered minwax before, but when I went to the hardware store I wasn't sure which kind to get, and this video answered that and more. This is a great tutorial video, thanks!
There are other tips on the channel in the playlist section as well, stuff to help with the strengthening and finishing of the model you are doing in FB, its simple stuff that makes a huge difference.
 

Hai-Lee

Old and Bold RC PILOT
#12
OK, for the sealing I actually use a cheap oil based polyurethane varnish. I wipe it on and off with a rag and the entire process is quite quick.

The exposed edges of the FB and other areas of exposed foam I simply wipe on white water-based wood glue.

When both are dry I just use any aerosol paint to finish the plane whereas if the FB is dark in colour or I grabbed the wrong colour for the job I use a grey primer to hide the unwanted colour and when dry use the proper colour to finish the job.

IF you want to bypass the sealing stage then you can use a straight application of water based aerosol paint BUT do so in light coats and keep the nozzle well away from the FB so that the propellant can evaporate fully before the paint reaches the surface of the FB.

I do no other work in the painting and no other materials, except for a little masking tape when painting stripes canopies and the like.

I also use the cheapest aerosol paints available here. No use investing in expensive paint if the plane is destined for the trash anyway!

Heck I even use the same cheap procedure on the planes I sell through the local retail model shop.

Have fun!
 

Hai-Lee

Old and Bold RC PILOT
#13
@Hai-Lee Which Minwax product do you recommend for sealing DTFB? It's hard to get FT foam in Canada so I usually just end up covering the lower surfaces of my planes with outdoor repair tape to keep the grass/snow moisture out.

I do have a HVLP gun that I could use with liquid for larger planes, but sometimes bombs are a lot more convenient than cleaning the gun.
I've been tempted before to spray Tremclad for its water resistance and glorious bright colours (plus I have gallons at hand), but it looks like it would add a lot of weight.
Any oil varnish is fine, (NO AEROSOL), and then any oil based enamel will give you a water proof and much more rigid bird than you would have though possible. The painted paper becomes quite rigid if done properly.

For the V. Large areas on bigger builds I sometimes I seal the FB using white water based wood glue in the same wipe on wipe off method. When dry this adds much more surface rigidity than just paint. I did this on a FB version of a Depron biplane and even though it was too heavy ultimately the crashes were all minor and the flat panel wings NEVER broke of crumpled even when flown at full throttle, or dived, into a tree.

By all means find out what is available locally and CHEAP andf work out your own procedure around that. Use cheap materials because the extra money is best saved for your next build.

Have fun!
 

BATTLEAXE

Well-known member
#14
For the V. Large areas on bigger builds I sometimes I seal the FB using white water based wood glue in the same wipe on wipe off method.
Do you mix it 50/50 or do you apply it straight on then wipe off, no wrinkling or delamination? I want to try this.
 

Hai-Lee

Old and Bold RC PILOT
#15
I use the glue straight because extra water soaks into the FB and actually warps and weakens the paper's structure.

Should you want VERY strong do the wipe on wipe off a few times with time to dry in between. Each successive coat can be thicker BUT note this; You need to coat both sides equally or clamp the sheet flat so that any excess glue and drying time does not cause the sheet to cup or bow.

If you find the sheet does want to warp a little paint the drying glue on the drier side with a little water to slow its drying and even up the tension.

When doing a folded up plane the warping is non-existent unless you get really heavy handed with the glue and soak the FB in it.

Practice on a few scraps first and you will see the strengthening effect. In addition once dry you can paint it with almost anything.

Have fun!
 

BATTLEAXE

Well-known member
#16
I use the glue straight because extra water soaks into the FB and actually warps and weakens the paper's structure.

Should you want VERY strong do the wipe on wipe off a few times with time to dry in between. Each successive coat can be thicker BUT note this; You need to coat both sides equally or clamp the sheet flat so that any excess glue and drying time does not cause the sheet to cup or bow.

If you find the sheet does want to warp a little paint the drying glue on the drier side with a little water to slow its drying and even up the tension.

When doing a folded up plane the warping is non-existent unless you get really heavy handed with the glue and soak the FB in it.

Practice on a few scraps first and you will see the strengthening effect. In addition once dry you can paint it with almost anything.

Have fun!
Thanks for the tip, I have way more glue then Polyurethane so that helps
 
#18
I have had good results spraying any of the Testor Enamel paints. They are pricey but foam safe. This is a depron Dog House out of foam I sprayed with Testors last week. You could just use t on the exposed foam as a primer and go to a cheaper paint on the rest. The wood detail I brushed on with acrylics from Hobby Lobby.
Thanks for the tip! I will have to give this a try when I get my LVLP gun. =)
 
#19
Just wanted to share another tip I got. Someone on the unofficial flitetest discord suggested I try off the paper in the shape of the servo and remove the foam, then cover that space and put plywood in it, after that glue the servos on. This is suppose to eliminate the issue with the paper delaminating and adds some additional stiffness to prevent the foam from flexing under high load/stress from the servos.

I will probably try this on my next build. This time around I just glued some wide popsicle sticks in before the servos to try and distribute the load over a wider area. They weigh less than a gram each and should be strong enough to do the job. I didn't remove the paper however, as I made a mistake and didn't cut the paper off where I am placing the servos before assembling the fuselage, and don't have the room to work to get it out safely.
 
#20
I just discovered that Testors is owned by Rust-Oleum, which makes me wonder if they offer some of the enamel paints at a cheaper price under the Rust-Oleum brand/label. I found this oil-based enamel primer which sells at Lowes for about $9 a 32 fl.oz can for example which I am curious to find out if it will do the job: https://lowes.com/pd/Rust-Oleum-Pro...or-Paint-Actual-Net-Contents-32-fl-oz/3069751

This said, I think I will have a pretty interesting set of primer experiments to try out when I do get my paint gun. =)
 
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