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Author Topic: Lots of pin holes and runny dope.  (Read 1345 times)

Offline Ty Marcucci

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Lots of pin holes and runny dope.
« on: April 21, 2018, 05:58:59 PM »
 D>K
« Last Edit: August 30, 2018, 02:51:42 PM by Ty Marcucci »
Ty Marcucci

Offline billbyles

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Re: Lots of pin holes and runny dope.
« Reply #1 on: April 26, 2018, 04:38:40 PM »
Last week I applied the first coat of white dope to my Road Runner, only to have a lot of pin holes show up and the dope was runny. Too thin, nope, over spray nope. Just in a few places and it started to separate and look runny.  I have had this dope for long while and no telling how much is has been thinned, etc. So with a new never opened can, I repainted the model. After a three day sanding off of all the previous paint. In some areas I even got down to bare balsa and instead of using carbon fiber, went back to tried and true silk span. Now it is "smooothh" y1.   Today I removed all the paint from the cowl and wheel  pants.  I used paint remover, which really stripped the dope.  The appearance of pinholes in carbon fiber is not all that common. but seems to happen time to time. I  think from now on I'll stick with silk span.
I think I have had this kit for at least ten years and finally got to it.  H^^

Hey Ty,

I gather that the pinholes showed up over the carbon fiber veil and that you sprayed the dope on.  I have found that when filling a porous material (carbon fiber, fiberglass, full-scale covering materials such as Poly-Fiber Dacron, Ceconite [also Dacron], or any other material with visible pores it is necessary to brush on the first few coats of dope or Poly-Fiber material to avoid pinholes.  Spraying it on will simply not fill the pinholes.  Also, this is stated in the Consolidated Coatings manuals for covering.  Some say that you can get away with spraying the first coats on but I have not, in more than 30 years of covering full-scale airplanes and more years with models, found it to be true.  In fact, even after brushing on the first few coats I have had pinholes show up and the only way to fix them is to go back to the brush method for one or two more coats then spraying can continue.
Bill Byles
AMA 20913
So. Cal.

Offline Randy Powell

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Re: Lots of pin holes and runny dope.
« Reply #2 on: April 28, 2018, 01:03:07 PM »
Contamination.
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 Randy Powell

Online Brett Buck

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Re: Lots of pin holes and runny dope.
« Reply #3 on: April 30, 2018, 10:28:31 PM »
HI Bill. I did brush on a few coats, but they may have been too few.  Maybe I should have used more filler on the damn stuff.  Any way, I removed the carbon and went back to silk span on the part that gave me fits.  H^^

   The pinholes in the dope or paint over carbon fiber (presuming you brushed it as Bill indicated) are probably the result of the sizing that holds the fibers together. There are at least two types of the very light carbon mat, one looks very uniform and smooth, the other very uneven and kind of clumpy. The latter is the kind you want, the former has a lot of sizing which is incompatible with the dope.

    I had a lot of problems with pinholes on my second Infinity which were not entirely solved, and it looked terrible in spots. This was using the "uniform" carbon. After researching the issue for a while, the same sort of problem seems to happen with fiberglass parts on pylon racers (also judged for appearance, or at least they were at the times). Their solution was to put on a skim coat of polyester filler - Bondo or HobbyPoxy PFC - work it into the holes, then sand it off smooth. If you wait until the primer is applied, there are only a few of them left, so the net weight increase is negligible, and the Bondo will be stable with any of the subsequent overcoats, and tiny red dots will easily disappear with the color.

    Brett

Offline Randy Powell

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Re: Lots of pin holes and runny dope.
« Reply #4 on: May 02, 2018, 02:39:53 PM »
So, contamination.
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 Randy Powell

Online Brett Buck

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Re: Lots of pin holes and runny dope.
« Reply #5 on: May 02, 2018, 02:52:38 PM »
So, contamination.

  Yes, sort of, if you consider something that was intentionally applied to hold it together that happens to impede paint adhesion to be "contamination".

    Brett


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