How to make a hollow fiberglass object.

[CDK]

Make a styrofoam core and paint it with pva.
Apply the first glass layers on a warm day so the resin cures fast.
The polyester fumes will ignore the pva barrier and dissolve (nearly) all styrofoam and maybe react with it too, because the sliver left is hard as stone.

This was an object I cut open because it was too long: I had no idea this had happened.

Applications? Making fiberglass tanks perhaps. The remaining bits of polystyrene cling to the inner walls as snowflakes and can be easily washed away with acetone.

[daiquiri]

I love these great casual discoveries!

Thanks for sharing it with us.

"TRIAL and ERROR"

..the unwritten book, and still the best tutorial.

Thanks Cornelis for sharing.

[SamSam]

30+ years ago I made a banjo case using polyester and Styrofoam. I wrapped the foam in duct tape and pva thinking that would work but it didn't. The foam melted as the glass cured so the case ended up usable but deformed. All that was left of the foam was a ball the size of a golfball.

What you're proposing is basically the lost wax process in a different medium, maybe it's already obscurely used in areas of manufacture. Maybe the lost wax process would work for fiberglass.

As you suggest, it seems it could be useful making seamless tanks and things. If epoxy was used, there would be no danger of the mold deforming and you could still dissolve the foam with styrene fumes and acetone.

[CDK]

SamSam, my object turned out well, probably because the resin cured very fast when I made it and it wasn't that large.

I was surprised to find the core gone and briefly thought about possible use for this procedure. But you are 100% correct: using epoxy and washing out the core with acetone is a less risky operation.

[SamSam]

Yes, the core gone in my banjo mold got me to wondering about my job building boats. A small operation with no protections.

[Stumble]

There is actually a very nice article on West Systems web page about this process.

Their recommendations are to use any type of cheap foam that is disolvable in actetone / solvent of your choice, shape the foam, add the glass/epoxy, then throught a weep hole add the solvent. I have used it a few times for one off parts, and it really does work well.

[AndrewK]

This is a common practice for making custom FRP exhaust components like water locks.

[LarryMcI]

Styrafoam cores work well for large objects too. Here's how we built the mast-tube for my boat:
http://sedation4.com/mast-tube.html

[tunnels]

Now thats what i call using you noodle !! very clever !!