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Cowl with zits

wirejock

Well Known Member
Ever seen a cowl go through puberty and get some serious acne zits? Click on the photo and zoom.
Go ahead and laugh. I did.
Long day of work gone. Oh bother. At least it's easy to find all the pinholes. Apparently the air in the pin holes off gassed. Curing room must have been too warm.
20220119_184132.jpg
 
I thought I did. Clearly I need an industrial size squeegee! Round two after this mess gets sanded off!

What are you using? Some folks recommended sheetrock mud, but that didn't work very well for me <I still had a bunch of pinholes>. I went back to an epoxy wipe.
 
One thing that might help is heating the shop, letting the cowl soak in the heat, shutting the heat off and doing the epoxy. As the shop cools it tends to draw the epoxy in to the pinholes.

Haven't tried this with my cowl, but it definitely works with wood parts (on a previous project).

Dave
 
Epoxy

What are you using? Some folks recommended sheetrock mud, but that didn't work very well for me <I still had a bunch of pinholes>. I went back to an epoxy wipe.

Same as every other part I've done.
Skim with neat epoxy.
Cure 30
Skim with neat epoxy
Cure 30
Skim with micro.
I probably wasn't forcing the epoxy into the holes. Should be easy to find them after sanding.
 
Are you using a steel squeegee? It can apply more pressure on the edge, and leave a thinner layer to sand.

But thinking, I don't think it is the root cause. I recall seeing some acrylic tables (or maybe it was a cooking show) being poured and they took a torch and waved over the top to bring out the air bubbles. I wonder if a heat gun could do that on an intermediate pass (well before the epoxy has begun curing) to identify any voids remaining. Just a wild thought.

Also, my painters just sanded all my glass down to the weave anyway.
 
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What do you mean by “cure 30”?

Are you using an elevated temperature cure? Fiberglass skins over core are notoriously porous, not just on the surface, but though the thickness too. It looks like you used a heated cure and air is being forced out of the core cells, bubbling up the epoxy and micro.
 
Looking back at the photo

The bubbles are only over the honeycomb core, and not over the solid laminate, which supports the idea that air is being forced out of the core cells if heated.
 
Ever seen a cowl go through puberty and get some serious acne zits? Click on the photo and zoom.

I've never seen anything like that...except when warming wet micro to make it flow, rarely a good idea.

Micro is for contour. Contouring (shaping ) should be long gone done before any applications of neat epoxy. Heck, micro brings new pinholes to the party, as it almost always traps some tiny air bubbles during mixing, and the glass bubbles themselves are hollow. Sand them open and each forms a microscopic pinhole.

In order:
1. bare glass structure
2. dry micro, block sanded to bring contour within a few thousandths of the desired surface profile.
3. neat epoxy to seal the surface, sanded just enough to remove drag marks and orange peel. Do not break through.
4. Two cross coats of a good epoxy primer. Use a color which will contrast with the next material, which for something like a cowl is...
4. Acrylic urethane high build primer surfacer, sprayed before the epoxy primer crosslinks. Block sand wet. This is where the surface becomes optically flat...no waviness to be seen in the finished, painted surface. Block until the contrasting epoxy primer starts to show through the high spots. If still bridging lows, shoot two more cross coats and sand again. Repeat until perfect.
5. Now it ready for paint, or the paint shop.

Blocking high-build requires fewer rounds if the earlier micro work was done right, and it takes less micro if the structural glasswork was done right, which usually means time spent on perfect forms and molds is time well spent.

Note acrylic urethane high build, not polyester. Polyester primer-surfacers were once the body shop standard, and there is a lot of it still used when cheap and fast are priorities. It very often does not stay stuck on epoxy glass, in particular the concave places, the only variable being how long it takes...months or years. Shrinkage and big blisters where it releases. The VariEze guys lived that one years ago. No point in re-inventing the square wheel.
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School of hard knocks

People ask me where I learned all the stuff I know.
Welcome to the "school of hard knocks".
Been there done that. I did laugh and cringed cause I hate glass/epoxy pimples.
Everything looked so right and then Bam it isn't.
Art
 
Popped the pimples

Seems like there's a show about popping pimples. Ok. Joking aside.
Pimples have been popped. Top cowl is sanded. Huge job I do not want to repeat.

What should I do next?
I think a new application of micro on one section is a good test. I don't want to sand the whole thing off again if it doesn't go well. Sand the micro to shape then skim with neat epoxy to seal.

Any input appreciated.
 
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