Learned a Trick for F/G Pinholes

Working on the interior of my 10's F/G top, which has lots of pinholes in addition to numerous other defects. I asked Barry at SPI for his advise. He said the key is to seal with the first coat with epoxy primer so the subsequent coat's solvents don't soften up the base coat and just keeps soaking in.

So, I put down two coats of epoxy primer and let it cure for 24 hours (the wait is a key part). 24 hours allows the epoxy to cure enough that it is imprevious to solvents and therefore locks in the base coat's stability. You could see the thousands of pinholes after the epoxy primer was on. However, they were concave depressions of the epoxy, which had bridged the hole completely. The SPI epoxy primer has enough thickness, or other properties, to bridge and not soak in. After 24 hours, I shot 3 coats of 2K high build urethane primer. Two probably would have been enough, but I had a variety of poly filler imperfections and 80 grit sanding scratches to get rid of.

After sanding tonight, it is perfect. Not a pinhole to be found or weave to be seen.

I know many struggle over this, so thought I would pass it along. In the past, I was putting the filler on too soon after the Epoxy primer and struggled with seeing the F/G weave through the paint.

Larry

The EZ community treats pin holes with an ?epoxy wipe?. The micro surface will have thousands of pin holes so we use West epoxy, pour it on, then try to squeegee it all back off while covering the entire area. Let it tack up, repeat 2-3 times. Final sand is done prior to primer, if you break thru the epoxy coat anywhere pin holes are possible and inspection or retreat is needed. The process works well.
I used SPI?s epoxy primer also, good stuff!
Tim Andres

I wonder about the long-term durability of bridging the pin holes rather than filling them up. Thermal stresses or internal pressure from moisture/pressure changes might eventually cause the bridge membrane to crack. What happens at 18,000 ft with a differential pressure of a half an atmosphere?

Epoxy thinned with acetone will go inside each pin hole and displace some of the air bubble inside it. squeegee that around a few times and the pin holes are mostly filled, not just bridged. Or use Jeffco 3114, which is already really thin.

Good composite parts can be made with no pinholes, by using an in-mold primer and a tie-coat of thickened epoxy before the first ply is laid into the mold. It does mean you have to finish the layup and get it vacuum-bagged and into the oven before the tie coat cures. Too bad our cowling fabricator doesn't do that.

Looks like they do now.

All the current stuff seems to have a primer/gel coat in gray with no detectable pinholes.

scsmith said:

I wonder about the long-term durability of bridging the pin holes rather than filling them up. Thermal stresses or internal pressure from moisture/pressure changes might eventually cause the bridge membrane to crack. What happens at 18,000 ft with a differential pressure of a half an atmosphere?
.

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I have no concerns with this. The epoxy primer is tough as nails once it has cured for a month or so. Further, there are several coats of urethane primer on top of it, plus the top coats. Also the epoxy primer seals the F/G so moisture cant get in.

Larry

mike newall said:

Looks like they do now.

All the current stuff seems to have a primer/gel coat in gray with no detectable pinholes.

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They do. I got my finishing kit parts a couple of months and they appear to be quality parts, done properly. There should be no pinhole issues. I haven't taken them for a carefull examination in the sun for weave print through, but it looks like they might finally be using proper top F/G layer to eliminate the weave print through.

The old green top from 10 years ago was a completely different story.

Larry

tim2542 said:

The EZ community treats pin holes with an ?epoxy wipe?.

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The result of experience. In forty years the EZ community has already tried every shortcut and filler imaginable.

The SPI recommendation Larry outlined is the same as that offered by a PPG rep many moons ago, although my interest was adhesion, not pinholes. I had seen polyester and urethane high builds peel some years after being sprayed directly on sanded epoxy/glass. The problem had included PPG K200, a very popular high build in the old days. The rep suggested a single cross coat of DPLF epoxy primer, followed immediately after solvent flash by K36 or K38, the products which replaced K200. The purpose was to use the epoxy primer as a tie coat, in effect gluing the urethane high build to the surface.

The SPI approach would do the same. It merely skips the prior epoxy wipe step. Like Steve, I'm not so sure about just bridging the pinholes; pressure change is an absolute, like gravity. On the other hand, I usually spray the backside of glass parts with epoxy primer, just so they look nice, and it doesn't fall off.

Time/work: epoxy wipe, sand, prime, shoot color. Or prime, shoot high build, sand, shoot color. Same thing really.

If you really don't want to do an epoxy wipe to fill the pinholes, I'd suggest Azko Nobel 28C1 prior to epoxy prime. It's what Lohle was repackaging as "WonderFil" a few years back. Note it is rubbed into the surface

https://005e08e88d620b24449f-d69587...ssl.cf3.rackcdn.com/product-document/28C1.pdf

tim2542 said:

The EZ community treats pin holes with an ?epoxy wipe?. The micro surface will have thousands of pin holes so we use West epoxy, pour it on, then try to squeegee it all back off while covering the entire area. Let it tack up, repeat 2-3 times. Final sand is done prior to primer, if you break thru the epoxy coat anywhere pin holes are possible and inspection or retreat is needed. The process works well.
I used SPI?s epoxy primer also, good stuff!
Tim Andres

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I tried this on my 6 and quickly became frustrated with the epoxy not curing and getting hard. I assumed the material was too thin to set up properly, but was probably user error. I ended up putting the SPI epoxy primer on by hand and using a squegee to press it in.

This new approach is just a heck of a lot easier and seems to work, especially when you need 2K primer anyways to smooth things out.

Larry

Using a flame torch

Learned a neat trick from a fellow who used to work on fiberglass boats for a living. When you spread the diluted epoxy mix per Vans instructions (I added a small amount of micro balloons), take a Benzomatic torch and lightly wave the flame across all surfaces. Obviously don?t hold the flame in place anywhere or you will damage the layup. The heat brings any bubbles to the surface right away and when the epoxy coating dries you will have zero pinholes. Don?t let the flame touch the fiberglass, just wave it over the surface about an inch away. A heat gun or dryer will push the slurry around so it will not work. The flame provides zero air pressure and you?ll end up with a very smooth surface.
When I took the airplane to the paint shop they were surprised to find zero pinholes during paint prep. Worked for me, but as always, your mileage may vary.

lr172 said:

I tried this on my 6 and quickly became frustrated with the epoxy not curing and getting hard.

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Yeah, that happens when a builder thins epoxy with a solvent. Thinning epoxy is A Bad Idea, but still recommended here and there.

This new approach is just a heck of a lot easier and seems to work, especially when you need 2K primer anyways to smooth things out

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I'm not discounting it Larry. As noted, it's exactly what I do when I want to spray K36 and block for flatness. It just skips the prior pinhole filling step, which has been done with epoxy, or 28C1, or drywall mud, all of which seem to be successful. This "new approach" is to simply bridge them, no fill.

Fly it 10 years and report back. Seriously. Everything works for a while.