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What happened to the finish on mt AT-6 ?

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Old 07-15-2013, 04:54 AM
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warbird_1
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Default What happened to the finish on mt AT-6 ?

i finished my byron at-6 last year , flew it a few times and once this weekend . it was HOT and humid also this weekend. Let me start by saying that i don't leave my planes uncovered out in the sun because dark colors get hot. After i flew the texan i took the wings off and started to pack up from the weekend when i noticed areas that was showing the construction through the finish. like where wood seams were etc. I used 3/4 oz. glass cloth , z-poxy finishing resin , klass kote primer and their epoxy paints . The finish was flawless , but now it looks terrible . even on the horizontal stabs you can see where i glued the tip blocks on because there's a seam line now as well as where the h. stab joins the fuse. and i even overlapped the glass cloth onto the fuse. also the dowel that holds the aileron control rod even can be seen now .Can anyone shed light as to why it happened and how to prevent it from doing it again?
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Old 07-15-2013, 05:24 AM
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Greg Wright
 
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Default RE: What happened to the finish on mt AT-6 ?

It's called SHRINKAGE of the foam. If it was me i would have used at least 1oz or even 1.5oz cloth on those foam surfaces.
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Old 07-15-2013, 07:25 AM
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Default RE: What happened to the finish on mt AT-6 ?

i didn't glass directly over the foam. the wing is sheeted with 3/32 basla sheeting . this might be one of the setbacks of glassing balsa sheeted foam wings , WB_1
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Old 07-15-2013, 09:54 AM
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Default RE: What happened to the finish on mt AT-6 ?

The epoxies shrink over time. What you can do in the future to help this is to spray a light coat of black laquer on the surface and sit it out in the sun to expedite the shrinking. I hae also heard of wrapping the cured parts in an electric blanket to keep them warm for a couple of days to shrink the resins.
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Old 07-15-2013, 11:37 AM
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Default RE: What happened to the finish on mt AT-6 ?

i can understand the shrinkage thing ,but i don't get why the seams would come though. i know that doesn't happen as much on built up ships. i may just sand them down and reshoot them this winter.
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Old 07-15-2013, 12:14 PM
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Default RE: What happened to the finish on mt AT-6 ?

All FRP is subject to "print through".
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Old 07-15-2013, 03:35 PM
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Default RE: What happened to the finish on mt AT-6 ?

Something to consider. If you fix and reshoot what's to say that the same thing won't happen again? So another possibility could be is to put panel lines on the plane. This would be an Visual Distraction and certainly scale. Real planes have pretty bumpy skins so you aren't to bad off.
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Old 07-15-2013, 05:52 PM
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Default RE: What happened to the finish on mt AT-6 ?

I don't know what kind of foam (open cell or closed cell) your wings are made of, nor the type of sheeting (balsa or light ply) in the areas that are problems. However, all of these materials shrink and swell depending on temperature and humidity. Most people are aware that wood products swell and shrink but plastics also move. Even with a good finish on the exterior which would prevent moisture penetration, the exposed cavities like wheelwells or even wingtubes can allow moisture in. Even in a completely sealed watertight vessel, changes of temperature can generate condensate. Wood products shrink and swell at different rates depending on the direction of the grain. Plastics change in a uniform rate in all directions...yet if a plastic component is 3 times longer than it's width, it will apear to grow proportionally more in the direction of it's length. Your pictures seem to show areas where different materials, different grain direction, or different thickness components are mating and in some pictures it looks like the butt splice of different sheets.

Like the above suggestions, thicker glass cloth and resin coat would create a firm shell that would not be as affected by the substructure changes...you built light, which has it's advantages but you have discovered the down side.
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Old 07-15-2013, 06:49 PM
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Default RE: What happened to the finish on mt AT-6 ?

What you have is shrinkage of the adhesives in all aspects of the build. What I used to do back in the day when I was building models that were glassed is this. First off use 1.5 ounce glass cloth. It gives you a more solid foundation for the finish over time. Next once the model is glassed you need to cure the resin for at least 30 days in a hot situation, and not the sun! Better to lock it in your car with th windows up for a month. It is like an oven and will fully cure the resin over the 30 days and handle most of the shrinkage. Next once you are primed and ready for paint, do it again. The primer shrinks! You will find all of this when you look at the model during these curing periods. It is very common to see more of this on gloss paint schemes.
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Old 07-16-2013, 03:43 AM
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Default RE: What happened to the finish on mt AT-6 ?

I don't know why people are fixating on the "adhesives." All of the materials used in our planes shrink and swell in different proportions, direction, and at different rates throughout the lifetime of the model. Exposures to extremes of temperature and humidity cause changes regardless of how old the model is or how long the resin has cured. Resin definitely shrinks during the initial cure, but once cured (assuming it was mixed appropriately) should be one of the most stable materials in the structure. I would add that the best integrated composites benefit from thorough penetration of the entire adhesive matrix and cross directional substructures...locking the different materials together. This is extremely difficult to do in a "built up model" since at least one side of your wing skin is "unfinished" allowing moisture penetraion to the balsa which is an extremely porous wood with directional grain. Light plywood is more stable and a better choice.

At this point, as for a solution, I'm agreeing with the panel line guy but for different reasons; Not so much a visual distraction, but rather thinking a heavier paint surface (a byproduct of doing panel lines) using a flexible paint (like latex) will better mask the movement underneath. The type of paint used has not been discussed but a common problem is expecting automotive finishes (which are hardshell finishes formulated for metal and hard plastics) to withstand the unstable substructure construction as well as the flight stresses and airborn flexing of a model airframe. Latex paints are used on houses because of their ability to flex with the materials beneath.

My last two cents on the subject
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Old 07-16-2013, 03:53 AM
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Default RE: What happened to the finish on mt AT-6 ?

i thought there was a considerable amount of time between the glassing and the final paint . 1.5 sounds heavy though the reason is sound to me. it just seems to me that alot of the warbirds i have seen didn't show this stuff. like i said theirs was built up vs. foam core stuff. FYI.. the foam used in the wings is most likely 3 lb. i just had some new cores cut for a corsair and i shed 3/4 of a pound of each by going to 1 pound . i guess i have to live with it. besides .. it flies so good
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Old 07-16-2013, 04:03 AM
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Default RE: What happened to the finish on mt AT-6 ?

looking back over my photos. i see it was at least 3 months from glassing to paint.
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Old 07-16-2013, 07:50 AM
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Default RE: What happened to the finish on mt AT-6 ?

You can also make a hotbox out of blue foam with a couple of 100 watt light bulbs and a dimmer switch to control the temp. This will post cure the resins and should take out much of the shrink. I am glassing the fuse of a pattern plane now and after the 2nd coat of resin, I wrapped it up in an electric blanket for 4 hours. i wil sand smooth, cook again, then prime and sand, then cook again.
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