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Old 06-07-2021, 08:52 AM
RV10Pilot RV10Pilot is offline
Join Date: Jan 2005
Location: Medford, NJ USA
Posts: 475
Default Repairing Poorly Countersunk Holes in Fiberglass

One would have thought that by the time I got to attaching the empennage fairing I would have known how to countersink. Not. I am using tinnerman washers to hold the fairing in place, but I know I can do better.Not. The nut plates are attached. How do you recommend I fill the poorly countersunk holes and start over.
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Old 06-07-2021, 09:04 AM
Ron B. Ron B. is offline
Join Date: Nov 2007
Location: Yarmouth, Nova Scotia
Posts: 2,630

If your not concerned about the rough hole (washer will hide ) then you can smooth out the counter sunk hole so that the washer sits better by having a piece of wood behind the glass clamped tight with the proper size hole for the center tip of your counter sink. This will stop the wandering of your counter sink.
If you want to repair the hole outside the counter sunk area you should grind a taper in your glass and glass over the hole and start over. I would try to glass from both sides, a little on each rather than all on one. Makes a stronger bond.
Thanks Ron
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Old 06-07-2021, 09:38 AM
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wirejock wirejock is offline
Join Date: Oct 2005
Location: Estes Park, CO
Posts: 4,772
Default Countersink fiberglass

Once fixed try a dremel and stone to countersink.
Run the dremel on a sharpening stine to shape the point to the correct angle.
Make a nice countersink. I usually stop shallow and use a hand deburring tool with a countersink bit to finish.
Larry Larson
Estes Park, CO
wirejock at yahoo dot com
Donated 12/01/2021, plus a little extra.
RV-7A #73391, N511RV reserved (3,000+ hours)
Empennage, wings, fuse, finishing kit done. Working FWF
I cannot be, nor will I be, held responsible if you try to do the same things I do and it does not work and/or causes you loss, injury, or even death in the process.
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Old 06-07-2021, 10:44 AM
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DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 10,356

slap some tape on the backside, wipe wet flox mix into the depression. cure, sand flush if needed, re-drill, countersink again.

Or just stop countersinking fiberglass. AN526's with nylon washers is the long term bulletproof solution.
Dan Horton
Barrett IO-390

Last edited by DanH : 06-07-2021 at 10:46 AM.
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Old 06-07-2021, 12:59 PM
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bjdecker bjdecker is offline
Join Date: Jun 2005
Location: Georgetown, TX
Posts: 956

Bond in some Tinnerman washers.
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Brian Decker
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Old 06-07-2021, 05:58 PM
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jrock836 jrock836 is offline
Join Date: Nov 2020
Location: Burleson, Texas
Posts: 329

To fill the holes, tape one side of the holes and the fill the hole flush with epoxy. Mix the epoxy thin enough so that it will flow into any minor cracks. Then re-drill in any manner you want. Your local marine supply store will have the epoxy you need, unless you just want to order online. Carefully dremel out the loose pieces of fiberglass on the edge of the holes before you fill.
Van's RV-12 #120566 - ELSA - N49DD
Spinks Airport, KFWS, Fort Worth, TEXAS!


Last edited by jrock836 : 06-07-2021 at 06:04 PM.
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Old 06-08-2021, 07:58 AM
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goatflieg goatflieg is offline
Join Date: Jul 2013
Location: Clarkston, MI
Posts: 836

Lancair sells abrasive countersink bits that are made for countersinking fiberglass. Worth the investment.
Martin J Filiatrault
Clarkston, MI
RV-8 #83507 - registered; final assembly continues; engine remounted.
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Old 06-08-2021, 08:26 AM
YvesCH YvesCH is offline
Join Date: Feb 2015
Location: Basel, Switzerland
Posts: 255

You can use Perma Grit Tools:

Cheers Yves
Yves - Basel, Switzerland
RV-8 #83458 HB-YUA
Donation 2020 done
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Old 06-08-2021, 09:37 AM
John Tierney John Tierney is offline
Join Date: Jan 2006
Location: Vonore, TN
Posts: 523

I have used this one with success, just very slow rotation:
John Tierney
Vonore, TN
RV-7A - N777JT Flying
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Old 06-08-2021, 09:42 AM
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Dan 57 Dan 57 is online now
Join Date: Sep 2018
Location: LSZF
Posts: 809

Yep, did the same as Brian, bonded the fairing washers in. Better looks and durable if properly done.
In your case RV10Pilot, it looks like the cloth wasn’t properly saturated with resin in the first place...
Life's short... Enjoy!

RV-6.9 #25685, slider, O-360-A3A (carb/dual Lightspeed II), MTV-12-B, HB-YLL owner & lover
RV-4 #2062 HB-YVZ airframe builder
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