I thought I'd post this since I've managed to find nothing on the topic so far. My "other hobby" is building ultralight aircraft and I have a fair bit of experience at the following technique, making cowlings/fairings ect. It really is easy, and can save a WACK of cash making custom panels/effects ect. PLUS, you can make custom pieces nobody else has rather than being just another bolt-on car. The only downside (big downside) is it is a LOT of time-consuming, very messy work for really not much return.
To make your own custom ground effects, bumper covers, scoops ect.
1st, you need a mold. Thats easy. Get the measurements from the car for the part you want to make, being VERY carefull with the mount point and corner measurements.
Get a bunch of styrofoam insulation and cut/glue it together to a very rough shape slightly larger than the part you want to make. Then use a rasp and/or coarse sandpaper to final shape it. If you want to use it for multiple
Now you have a couple of choices. You can give it a thin coat of drywall mud then spray the mud (after drying) with a cheap primer/laquer whatever and give it a good coat of wax for a release agent (turtle wax works fine)--OR, you can just cover it in wax paper for smaller parts.--just make sure it is waxed so your body panel will release.
Once the mold is done you are ready for your lay-up. I've developed a technique that I find results in strong yet light panels.
1st step is to give the mold a coat of epoxy (no glass yet) and let it kick. this will give you a smooth inner surface with no worries about filling or glass penetration. Once it has set up (I usually give it abouit 2 hours at 70 deg F ) sand lightly to scuff the surface and lay on the 1st layer of glass cloth dry, then work more epoxy thru it. repeat for a second layer.
I generally use 6oz bi-directional since its light, strong and easy to get.
Once the seconf layer is kicked, use epoxy to glue fender washers at all your mounting points as re-inforcement. You can also use strips of .020 to .040 aluminum to reinforce edges, coreners ect. Just make sure the faces are well scuffed for bonding and m0ost of the piece looks like cheesecloth from drilling 1/8" holes thru it (to allow the resin to form a continuous bond between layers).
After the rienforcements are in place, I lay in a layer of ALUMINUM window screen. Leave about 1/2 to 1" around the edges so it doesnt show. This stuff is incredibly strong in tension and adds a great deal of reinforcement to an otherwise flimsy piece. Epoxy the screening, washers, reinforcements ect into the lay-up. Scuff once its kicked.
Since epoxy is expensive (25 bucks for an 8 oz kit) I now save a bit by switching to polyester resin. It will bond fine, just make sure the epoxy surface below is well-scuffed and clean.
Use 1 or 2 layers of matt (as required) to build up any voids between the washers, screen, reinforcements ect. Once the resin has set in the build-ups, add 2 more layers of bi-directional cloth.
Finish with at least 2 coats of resin painted over the last layer of cloth to fill the weave and help smooth the surface. So long as you add new layers of polyester resin within 24 hours you dont need to worry about scuffing the surface below---it will chemically bond..
You can now remove your part from the mold, sand/bondo/spot fill and repeat and repeat and repeat
untill you have a smoothe surface. Prime it, trim the ragged edges to size/shape with a dremmel, drill out the mounting holes, re-prime and paint.
Its ready to bolt in place.
It sounds like a lot, but its possible to make a piece similar to a Z24 side skirt for around 50 to 75 bucks canadian. Not worth it if you just get a factory one from a junkyard, but loads better than paying 400 USD for somebody elses idea of a custom skirt. Last fall I built a cowling that retails for $2700 for under 300 bucks.
The best bet before making actuall car parts is to try out a few small "junk" panels untill you are comfortable with the technique. If you find 4 layers of glass plus the screen too light for your tastes, just add a layer or 2 of heavy uni-directional cloth under the last 2 layers of bi-directional. Just make sure the weave is well-filled before adding the thin stuff.
Sorry if this is in the wrong forum, feel free to move it if it is.
Rice.....Part of a balanced Pontiac diet.
