Thoughts on Fiberglass Rear-Skirt Design

[Bicycle Bob]

Originally Posted by stevey_frac

Fibreglass Questions: Since you guys have worked with it before.

Do i have to wait between applying layers? And how far will one of those kits from Cdn Tire go? Could I make one skirt with one box? or would it take multiple boxes? Do I need to have a core of some sort (i could probably use some thin plywood) or can i just schlep enough fibreglass onto it and it'll work?

Also, i was going to do a kind of raised flat panel, and then i wanted it to have like.. a 1 inch smooth section to rest agains the edge. Can i make the panel large enough to cover the the whole edge, then have a section that angles back to rest against it? It's kinda hard to explain...

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Where the wheel well is 'up' from that, and those flat edges rest against it. The problem is.. How would i get it out of the mold. Or am i going about this all wrong?

-Steve

A kit might be good just for the education, but I don't know what is in that one. The most common fiberglass cloth is 6oz per sq. yd "boat cloth" and you would want at least five layers, all mushed together while still wet. Cheaper fabric is a felt mat, sold, confusingly, by weight per sq ft. Two layers of 1.5 oz might be OK. Mat winds up with a lower glass content, but the thickness build-up with cheap resin makes it good for non-structural panels. Plus, it is less likely to show a pattern through the gel coat. I wouldn't bother with a core material on a first project. That is a way to get the lightest possible part, not a successful first project.

To get that edge shape, you'd want a two-part mold, and it isn't always easy to lay 'glass in a tight curve. A larger skirt overlapping the fender could have a much slipperier shape.

Polyester resin won't cure on the surface when exposed to air. This is handy when adding layers to get bonding without sanding. The final layer has wax added to float up and seal the surface. So always check for "waxed" or "unwaxed" labels, or buy the resin and wax separately. You would use unwaxed gel coat against a mold (with release wax - a separate issue) and let it cure before adding the cloth, using wax for at least the last layer. You can usually sand wax off easily as you rough up a cured surface for a secondary bond, but gelcoat might break up.

[stevey_frac]

Originally Posted by wyatt

Here is a "simple" method of building fiberglass parts. I think it goes with what bicycle bob had said, but instead of using the wood, you use foam. You can fiberglass over both sides of the foam and have a thick part, or do one side and mechanically (scrape, hack, scratch, sand) remove the foam.

Can I not use the foam as a mold to make many out of?? Or do i have to make a more complicated mold for that? I was planning on making it out of that foam board insulation, glued together. The difficulty would be in removing the plug after the fiberglass had set.

[Bicycle Bob]

Sure, you could build a surface of wood or foam splines, and 'glass over the outside to hold the shape, and then do the inside to get a sandwich-core part. That way, you don't have a mold left over. The weight savings from the core will probably disappear into filling and fairing the surface. The wood core would want expensive, hard to sand epoxy, so the urethane foam sounds better, if you can handle it neatly. Using a thick build-up and then removing the foam is also a valid proceedure, but it tends to require more fiddling around the details. Fortunately, it is very easy to add bits to a 'glass part.

[stevey_frac]

Originally Posted by Bicycle Bob

Sure, you could build a surface of wood or foam splines, and 'glass over the outside to hold the shape, and then do the inside to get a sandwich-core part. That way, you don't have a mold left over. The weight savings from the core will probably disappear into filling and fairing the surface. The wood core would want expensive, hard to sand epoxy, so the urethane foam sounds better, if you can handle it neatly. Using a thick build-up and then removing the foam is also a valid proceedure, but it tends to require more fiddling around the details. Fortunately, it is very easy to add bits to a 'glass part.

You guys are honestly about a 2 leagues above me with this stuff.

Ok. Let me throw this at you for a procedure, and you tell me if it'll work.

1) Build up a foam block by gluing together a bunch of sheets of foam insulation
2) Cut away excess material until the block is the same shape as the fibreglass part i want to make
3) Sand the crap out of it
4) Coat the foam block, with wax / release agent / something to make it not stick to the fibreglass
5) Put on 5 layers of cloth/resin. I think the Canadian tire stuff is all waxed.
6) Separate part of block
7) Sand the crap out of the part
8) Paint, mount, and enjoy increased fuel economy
9) ?????
10) Profit

Only thing i don't have worked out in my head, is how to mount it after it's done that dosen't involve punching holes in it. Somehow it seems wrong to go to all the effort to get a professional looking cover, and then put a screw through it.

The Canadian tire kit comes with 1L of resin, fiberglass mat, and hardener.

You (bob) seem to indicate that if i don't have enough, i can go out, buy another kit, and add layers. Did i get that right?

[Bicycle Bob]

Hmm, I was chasing a linkback, and found this old unanswered query.

Building up a foam block is popular. The glue lines can be a problem when sanding, but latex-based contact cement, preferably aerosol, is not too bad. To create a big blob at the same price, you can mix Part A and Part B by sloshing them around in a garbage bag which becomes the rough mold.

Snap-off blades make great carving tools but dull very quickly. Sanding is good. You can buy urethane foam, which is impervious to polyester, and use scraps to sand the rest, as blocks that shape to the correct contour.

The foam surface must be filled smooth to release a mold. Candle-wax (Parafin in America) might work, applied with a hair dryer, but I've never heard of it. I have used a mix of microballoons, wall repair compound, and latex paint, which dries to the same sanding-hardness as insulating styrofoam as a filler and top coat. It, in turn, has to be sealed and polished, using either Shellac or Polyvinyl Alcohol (PVA - also handy as a water-soluble mold release)
Then it gets waxed with mold release wax, which is very thin and hard, to preserve details, good or bad. 3 coats are always used on a new mold, along with muttered incantations and/or other safeguards. (OTOH, I knew a guy with a whole shelf full of oddball resins, with the recommended release agent for each. One day, he used Pledge furniture polish on a countertop, and a sample of each resin released just fine.) A "soft" master made of foam and filler will break up during release, so it is probably better to just use regular mat and gel coat for a surface. You can protect styrofoam from a 1st layer of polyester with a layer of masking tape, and use easy-to-sand, quick-to-patch polyester all the way to the 'way-easier-to-polish final surface. It can be good practise with the stuff, too.

Usually, we make a master model of a part, and pull a mold from that, to get the shiny side on the outside of the final part. For a shallow shape like a fender skirt, it might be OK to use a male mold, and finish the outside. With a re-useable mold, you might be able to do the other side by starting with a thin layer, and then turning it inside out. Then add a second layer to give final strength, and tape it on while still "green" to take out any warpage. That won't work for a flanged edge, of course, just a spoon shape.

To create hard points for mounting hardware, you can add layers to make a thicker pad, and then encapsulate a bolt head or nut, visible only from one side. The layers of cloth or mat with the hole have to be well bonded, and spread out to a long tapered edge.

Adding material from a second kit sounds fine. Just remember to thoroughly sand off any wax before making secondary bonds, and leave it rough from 100 grit. While epoxy is quite wonderful, and on average is tougher than polyester, you should not be relying on the resin for strength in any composite part. It is only there to keep the strong fibers lined up.

[3-Wheeler]

I noticed that Bicycle Bob is advocating making the skirt from about five layers of fiberglass cloth, and this would work fine....

But there is another way that I have successfully made various shapes over the years.

Make the entire shape of your skirt with polystyrene foam that is bought for home insulation.

As you mentioned, cut/carve/shape and so on to get the shape you are after. When doing this, try to keep the minimum thickness of the foam at least 1/4 inch thick in the thinnest spots, and up to 1 inch thick where you can.

Now when you have the shape exactly as you like it, you can apply the cloth with your choice of resin, but if you use Polyester, then you must coat the foam with something like Latex primer paint before adding the Polyester resin.

For a part this small, I would pay the extra and use West System Epoxy resin, which does not eat the foam in the process, so no painting is necessary before adding the resin.

Add one layer of 6 ounce glass to the outside and inside for strength. No need to add more layers, as this will be plenty strong.

Add wood blocks inside the foam where the skirt attaches to the car, and add several more small patches of glass in these areas for support.

Almost forgot... Making a foam/fiberglass composite will result in lower weight of the overall panel, since there are only two layers of glass, instead of multiple layers when making a fiberglass model (at least five for strength) from a plug (male or female), depending on what surface you want the smooth finish. In the case of the polystyrene filler, the extra thickness of the filler adds hardly any weight, as the density of the foam is low compared to the weight of the fiberglass and resin.

Also keep in mind when doubling the thickness of a panel, the buckling strength goes up four times for this thickness increase, with only a small increase in weight. Formula race cars use this same construction technique, except they substitute aluminum honeycomb for the foam core, and carbon fiber for the fiberglass cloth.

Jim.