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  #1  
Old 04-15-2018, 06:13 PM
JDA_BTR JDA_BTR is offline
 
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Default Antenna sealant and paint

Garmin says to use Dow Corning 738 to seal around the ga35. Ive read here that silicone messes up paint applied later. Does the painter pull the gps antenna to paint? Dues the paint prep make this a non-issue?
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  #2  
Old 04-15-2018, 07:52 PM
KeithB KeithB is offline
 
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Quote:
Garmin says to use Dow Corning 738 to seal around the ga35. Ive read here that silicone messes up paint applied later. Does the painter pull the gps antenna to paint? Dues the paint prep make this a non-issue?
Silicon will definitely mess up paint (according to my painter). I mounted antennas without sealant (and flew for almost a year), removed them before painting, remounted and then sealed with Dow Corning 738.
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  #3  
Old 04-15-2018, 09:36 PM
tgmillso tgmillso is offline
 
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A polyurethane sealant will work fine for painting over (eg. Sikaflex 221, 252, Pro, really anything so long as it is PU). You may even find a Modified Sili Polymer that works, however these can be a little more fussy regarding paint. If I was in your boat I'd cut the holes and install the require doublers but wait until I had painted before I final install the antenna, unless there's a particular reason to do the final install beforehand.
Tom.
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  #4  
Old 04-16-2018, 02:33 PM
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Erimo Erimo is offline
 
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Quote:
Originally Posted by JDA_BTR View Post
Garmin says to use Dow Corning 738 to seal around the ga35. Ive read here that silicone messes up paint applied later. Does the painter pull the gps antenna to paint? Dues the paint prep make this a non-issue?
I asked myself the same question. Some people mount their antenna with only the cork seal. I choosed this :

https://www.amazon.fr/gp/product/B00...?ie=UTF8&psc=1

It can be paint.
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  #5  
Old 04-16-2018, 09:27 PM
JDA_BTR JDA_BTR is offline
 
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Makes me think about buying a little bit of gasket material and installing with a thin gasket. Later to paint I can pull the antenna and the gasket, then final install can be with the sealant.... My transponder antenna came with a little gasket....
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  #6  
Old 04-16-2018, 10:18 PM
rv7charlie rv7charlie is offline
 
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Don't know why one wouldn't want paint under the antenna; if it gets changed out later during an upgrade, the new one might be a different shape. And you really don't want paint on the antenna itself.
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  #7  
Old 04-17-2018, 05:14 AM
JDA_BTR JDA_BTR is offline
 
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I totally agree. Part of the reason I didn't rush to put the sealant on....
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  #8  
Old 04-17-2018, 05:22 AM
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Walt Walt is offline
 
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Originally Posted by rv7charlie View Post
Don't know why one wouldn't want paint under the antenna; if it gets changed out later during an upgrade, the new one might be a different shape. And you really don't want paint on the antenna itself.
Perhaps AC 43-13, 2B Chapt 3 can shed some light on this:

7. ANTENNA BONDING.

a. The electrical bonding of the antenna to the aircraft surface is extremely important. The conductive skin of an aircraft is an electrical part of the antenna system. If an antenna is not properly bonded to the aircraft, its pattern may be distorted and nulls in coverage may appear.

b. the electrical bonding of the antennas to the aircraft skin of a metal aircraft is best accomplished by direct metal-to-metal contact of the antenna base to the skin. A resistance of no more than 0.003 ohms between the antenna base plate and skin should be achieved.
NOTE: To achieve this electrical bonding, the aircraft paint in the mounting area will need to be removed and the surface covered with an oxide film (i.e., aluminum conversion coat) to protect aluminum against corrosion in accordance with MIL-C-5541B.

C. An alternate method for providing electrical bonding to metal aircraft skin is through the antenna mounting screws, which attach to a backing plate inside the aircraft, making electrical contact with the backside of the skin. To ensure good contact, remove any interior paint in the area where the backing plate is placed and coat this area in accordance with MIL-C-5541B to minimize corrosion.
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Last edited by Walt : 04-17-2018 at 05:27 AM.
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  #9  
Old 04-17-2018, 06:54 AM
rv7charlie rv7charlie is offline
 
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There's not much of this horse left to beat, but here goes.

While the FAA's regs & AC43 should always be consulted, that doesn't say they are always perfect.

Bonding of the antenna to the skin requires a tight, gas tight joint (and I'm not talking about the sealant around the edges). Try this experiment: smear some type of 'witness' paste (conductive paste, or even grease or vasoline could be used) on the base of an antenna, and then mount it to a 'skin'. A scrap of aluminum sheet would do fine for the experiment. Tighten the screws to spec. Now remove the screws, pull the antenna loose, and observe where the antenna base actually contacted the aluminum sheet. You'll see that it only contacted the skin reliably at the points directly under/around the screws.

A star washer between skin and nut on each screw, and another under the head of each screw, will provide more effective bonding than any level of bare skin.

And that big area of bare skin under the base is an invitation for corrosion; moisture can enter the area through the screw holes in the skin from inside the structure.

Charlie
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  #10  
Old 04-17-2018, 07:33 AM
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Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by rv7charlie View Post
SNIP
A star washer between skin and nut on each screw, and another under the head of each screw, will provide more effective bonding than any level of bare skin. SNIP
Charlie
Exactly correct. Hopefully we donít have a bunch of people scrapping off paint to comply with guidance that is simply not applicable to our applications.

Carl
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