Fiberglass basics - Ack! Your tips needed

[bennelson]

I haven't been able to find sheets of rigid urethane foam anywhere. The only thing I could seem to find for urethane foam was was either expanding foam in a can (Great-Stuff) or soft foam used for upolstery.

The idea of using rigid foam is that it's affordable, and easy to cut and carve into either large flat areas or not curvy parts.

Maybe I should price out soft yellow foam? I bet it could be put over some sort of framework, and then fiberglassed over, and have the framework removed.

[Ryland]

I like using wood instead of foam, bass wood is really light, not as light as balsa wood but close and much cheaper! 1/8" thick birch ply wood also works well, ceder strip canoes are made kind of like this, on it's own the wood is not strong enough and the fiberglass alone would be much heavier but an epoxy/fiberglass/wood composite is really strong! it's also easy to work with and easy to shape and if you need to fill an area mixing epoxy resin with saw dust works well for filling in voids.

[3-Wheeler]

Originally Posted by jamesqf

Polyester resin does not eat through urethane foam. (It's a yellowish-tan color, rather than the pink or white of styrofoam.) Couple of other advantages, too. One is that you can build up areas with urethane sealing foam ("Great Stuff" is one brand), another is that it bonds really well with urethane glue ("Gorilla Glue").

James,

I must say that my experience with GreatStuff comes with some cautions.

It works good for areas that need filling, but it keeps expanding for about one month afterwards. If this is OK, then by all means go ahead and use it.

Gorilla Glue is basically the same product except that it is much thicker and expands at a much lower rate, but IT STILL EXPANDS.

I had trouble with Gorilla Glue on the tail around the side mounted rear facing lights, as it kept expanding and bulged the foam in the area.

If I had to fill an area and really want the finished product to be mechanically stable, then I would use either pink foam and/or epoxy resin mixed with micro-ballons as the filling agent.

Jim.

[CigaR007]

Keep in mind that expanding foam is affected by the temperature. A low temperature at the time of application will reduce its expanding properties.

[Daox]

Originally Posted by bennelson

I haven't been able to find sheets of rigid urethane foam anywhere. The only thing I could seem to find for urethane foam was was either expanding foam in a can (Great-Stuff) or soft foam used for upolstery.

The idea of using rigid foam is that it's affordable, and easy to cut and carve into either large flat areas or not curvy parts.

Maybe I should price out soft yellow foam? I bet it could be put over some sort of framework, and then fiberglassed over, and have the framework removed.

Polyiso foam is available at the local menards. It is light tan/yellow and has a foil facing on it. It is a bit more expensive for this vs extruded polystyrene, but you're not talking much for 1/2". I think my 1" thick sheets were $13. I don't know why I didn't think of that earlier when we were talking about this...

This is the stuff I'm using to insulate the bottom of my solar hot water tank:

[HAHA]

Well - I'm a boatbuilder so I can submit some information on foam, glass, polyester etc.

Unfortunately, there is no cheap way out of the foam problem if you want a durable, water resistant and lightweight construction. Sandwich construction, i.e. core material with glass around, is used extensively in boat building. However, the foams, honeycomb, balsa or whatever, need to be of very high quality. If you are building a plug, you can get away with cheaper materials that will give you something with low structural integrity (like what you are trying now).

I'm situated in Europe so the products available here are not always the same as in the Americas. A good source of information and products is DIAB: Americas Home Page

For an application like yours, you will be looking at low density foam. I would not go below 40 kg/m3. Divinycell is one of the best products for this and it is very easy to form and grind to desired shape. Cheap it isn't but you will not use much anyway. If you can locate a boatbuilder, he can supply you with everything you need, including information. Performance boat forums are probably a better place to source information on building technique. There should be plenty of project threads with pictures etc. if you look around.

[YeahPete]

I have heard of people placing foil over the foam. And taping the foil in pace. They then wax over the foil. I am not sure of the type of resin used though. I will be in your shoes learning fiberglass hopefully in the spring.

[HAHA]

With foil over foam, you will not get a sandwich construction. The foam and everything under the glass/matrix will have to be discarded.

If you use a proper foam, like Divinycell, Airex, Balsa etc., the foam will be permanently bonded to the glas and constribute to the strength. In this case, you can use just a thin layer of glass (400 - 900 g) on either side of the foam to get a really strong and lightweight laminate.

If you want to make a cheap plug or mould, a trick is to cover the not-styrene-tolerant foam with plaster. This is cheap and harmless and can be shaped easily enough. Before putting resin and glass on it, make sure you wax properly or use mould release fluids.

[NeilBlanchard]

What I am going to try on constructing CarBEN EV5 is using waterproof "wood" glue with fiberglass over high density extruded polystyrene foam. It is cheap, fume-free, cleans up with water. It seems like it will be a good mix - the Titebond III that I am using stays somewhat flexible after drying.

[bennelson]

Thanks for all the responses!

Daox: I was digging through my pile of scrap materials, and there was a small piece of yellow foil board, like what you are in using. I said to myself "Ah ha! I bet that's urethane in there!"

NeilBlanchard: I did some searching on a prop-building forum and saw a demo of using PVA glue (plain white school glue basically...) as a coating over foam for building Iron Man armor. It looked REALLY good.

I'm planning on doing a test with white glue over foam and see how it works.

I DID do a test with aluminum foil, and it worked well for preventing damage to the foam, but as was said, it doesn't bond to the foam at all then. Would work fine for mold-making!