What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Repairing Poorly Countersunk Holes in Fiberglass

RV10Pilot

Well Known Member
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.
 

Attachments

  • IMG_4593.jpg
    IMG_4593.jpg
    329.3 KB · Views: 429
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.
 
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.
 
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.
 
Last edited:
Bond in some Tinnerman washers.
 

Attachments

  • IMG_1972.jpg
    IMG_1972.jpg
    337.8 KB · Views: 485
  • IMG_1975.JPG
    IMG_1975.JPG
    374.4 KB · Views: 414
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.
 
Last edited:
Lancair sells abrasive countersink bits that are made for countersinking fiberglass. Worth the investment.
 
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...
 
K.I.S.S.

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.

Yep...I did THIS ^^^^

Hardly miss the .00005 knots at cruise. :D
 
Yep...I did THIS ^^^^

Hardly miss the .00005 knots at cruise. :D

To help you understand why this works so well, understand that the Cotton Flox they are talking about when mixed with epoxy is rock hard. It also is cheap, light, and easy to get. If this is new to you...I suspect it is...then go for it. You wont mess it up. Just mix up a tablespoon of epoxy (stir of two minutes) in a paper cup, then add flox so its a constancy of between mayonnaise and peanut butter and then use a Hotel key card or credit card to wipe it over the hole you are repairing. You'll have it done in thirty minutes.
 
Thanks for all the help. Packing tape on the backside, wet epoxy flox mix. Coated the hole with neat epoxy and then applied the flox mix. A small piece of peel ply on the top and let it cure.

I drew a line from the center of the screw hole and marked 2" and 4". Put the fairing back on and then used the line and markings as a guide to drill the hole. Worked perfectly.
 

Attachments

  • IMG_4616.jpg
    IMG_4616.jpg
    264 KB · Views: 243
  • IMG_4617.jpg
    IMG_4617.jpg
    288.8 KB · Views: 271
Back
Top