Dealing with pinholes in fiberglass.

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Brad
Posts: 15
Joined: Thu Aug 31, 2017 1:17 pm
First Name: Brad
Last Name: Carter
City or Town: Shawnee
State or Province: Ks

Dealing with pinholes in fiberglass.

Post by Brad »

I thought I would describe how I deal with pinholes in the fiberglass parts for you non-fiberglass airplane builders. I’ve tried several methods over the years and this is the one that works best for me. First off you can’t “spray away”pinholes, all you will do is cause runs in your primer, they have to be filled. Start off with your parts contoured to the shape you want with a mixture of West System epoxy and micro balloons. Fill any “craters” with epoxy and micro balloons, this is about filling pinholes not craters. Sand the parts up to about 220 grit. Blow off the dust with an air nozzle. Spray on a medium to heavy coat of a high fill 2 part primer, I use Upol 2251. Now you will be able to see the extent of your pinhole problem. Likely you will have between hundreds and thousands of pinholes. Mix a small amount of primer, around 20 grams, I use weight for mixing the correct ratio of primer to hardener, in the small batches volume measurement doesn’t work very well. Then add micro balloons until the consistency of maybe honey. Now with a hard rubber squeegee work the mixture into the pinholes as best you can. You will have about a 30 minute working time so it may take a couple of batches to do something the size of a wheel pant. Try to develop a pattern so you don’t miss any areas. Let it dry, then dry sand most of it off using a rubber sanding pad and 220 sandpaper. Don’t sand into the fiberglass! Blow off the dust with an air nozzle. I don’t like to wet sand until most of the pinholes are filled. Hopefully at this point most of the pinholes will be filled. If you still have a lot of pinholes use the primer method again. Most likely you’ll have 20 to 30 left, for a small amount like that I use a red oxide, lacquer based spotting putty to fill them. It doesn’t need to be mixed and dries fast. Sand again with 220 but don’t overdo it, try not to sand into the fiberglass or you may cause new pinholes. Now spray another coat of primer and evaluate the pinhole situation likely there will be very few. Repeat the above until you’re happy, wet sand up to 360-400, spray on color and your done. The SPA fiberglass parts are pretty good, but there will be pinholes. I haven’t seen a vendor or builder produced part that doesn’t have pinholes. All this fiberglass work will make you appreciate the fact your building an aluminum airplane.
Brad Carter

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