Fiberglassing: Help me know what I'm doing!

OK I know theres been threads that covered this, but I'm not finding it using the search function, and its not really prop related but assembly related, so this would be the best audience.

I need to make a hollow 13" in diameter sphere out of fiberglass. The inside needs to be accessible so I will be doing two hemispheres and attaching them together.

I have what I think will be a good enough mold - those red rubber playground balls that you played 4 square with during recess in the 4th grade. Well, I did anyway.

What I was planning on doing was to cover the ball with a couple layers of saran wrap, and then glass on top of that. That way the resin wont eat away the ball.

This is my first experience with fiberglass, so I kind of don't know the deep details for everything. Which means I have a bunch of stupid questions to ask.

1) Is that plastic wrap idea going to work? It doesn't really matter to me if the plastic sticks to the resin or not, but I can put mold release on anyway (I was thinking some WD-40?)
2) The interior of the ball is (hopefully) not going to be exposed to the elements. Do I need to put a coat of gel coat on before glassing or can I just put some of the resin on?
3) The exterior of the ball WILL be exposed to the elements. Its going to end up on the top of a building and needs to weather storms. I would assume this would impact what sort of resin I would need to use, but I don't know much about them to pick. I was planning on just using this stuff: TAP General Purpose Epoxy: TAP Plastics but I don't know if I need something better. This is going to be painted, does that mean I still need one with better UV protection? Marine grade?
4) I need this ball to be non-conducting. It doesn't need to be insulating, but due to whats inside it, I can't have it conduct electricity. I didn't think this was even an issue with fiberglass, but on another project we were using fiberglass plate, and we discovered it was conducting (which was not good) so I want to make sure all bases are covered.


I think those are all the stupid questions I have now, but I'm sure I will have more later, maybe the others hiding in the shadows with similar questions can chime in too!

My thought is this: Skip the red ball and plastic wrap. Go online and buy an acrylic hemisphere at the proper diameter to use as a mold.

Use proper release: Wax and PVA. Laminate the inside, let set to gel-state and trim. Repeat.

Here is a trick for getting the two halves to fit nicely together: Cut a good size door in one of the halves, big enough to gain access to the interior. Then PVA the inside of the door and tape back into place. From the inside, fiberglass 0.5" beyond the edge and let cure. Once cured, remove door and sand the edge of the new glass to prevent cuts. You have created a lip for reattaching the door. Now tape the two hemispheres together and fiberglass the seam and let cure. Once cured, put some mud (catalysed resin mixed with fibers and cabosil to thicken) onto the lip, and press the door into place.

If you need access, to the interior, you can attach the door with (plastic) screws

clonesix said:

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My thought is this: Skip the red ball and plastic wrap. Go online and buy an acrylic hemisphere at the proper diameter to use as a mold.

agreed,

the time, hassle and expense you will have the other way will having you wish you just bought it already done. life is too short.

i used to do it your way but i got smarter.

-z

Hmmm. I looked at plastic hemispheres, but I didn't go with them because one of those taps out my entire budget, leaving no room for the raw materials. ($45 vs $10)

Clone6 - I think I like that idea for attaching the halves together. My initial idea left it much more open but might not have as good as a seal. The only thing I will need to look into is how often we'll need to get access to the parts inside the ball (this is going to be a weatherproofing shell for a satellite antenna)

In that event, I recommend a vinyl play ball, rather than the red rubber ball. The surface is smooth, and you won't need to use plastic wrap, just a good release.

Price of a acrylic hemisphere a concern? Don't scratch it and clean it up real well, and return it as it was the wrong size for your application.