I'm working on a plug for the forward "Main Ballast Tank" for my K-350 right now. I decided the shape I wanted to make would be a bit more aesthetic than the standard parabolic half cone shape. I decided to start with the blue foam board and go from there. I glued a bunch of 2 inch blue board together and rigged up a hot wire cutter using my welder (on the lowest setting) and a piece of stainless steel TIG wire for an element. I strung the wire across a frame resembling a bow saw with one end insulated from the frame of the saw. Each end of the wire is connected to the welder leads. To rough out my shape in the polystyrene blue board I just flipped on the welder and ran the hot wire through the stuff like a "hot knife through butter."
To glue up the blue board, I used paneling glue in a caulking gun tube. It glued well and cut with the hot wire good too, but it resists sanding and has to be cut out where ever I sanded in to it. I'll try the Elmers glue you suggest next time.
Once it was roughed out, I used Plaster of Paris to develop the shape farther. Plaster of Paris sets up fast and doesn't shrink as drywall compound does. Once my shape was closer to exactly what I wanted, I then switched to drywall compound. This goes on nice but shrinks a bit if put on heavy, and since it cures by evaporation, takes longer to harden. A heat lamp speeds it up though. The nice thing about dry wall compound is it sands really easily. It's far easier to fine tune the plug. Bondo takes much more elbow grease to sand down.
The plug I'm making is to make a mold from so it's surface will be what I get in the finished part.
After I'm satisfied with the plug, I'll shellac it, wax it and put on some mold release before laying up the fiberglass for the mold. After the mold is laid up and cured I can use it to make the finished part.
This method does involve the extra step of making the mold but I'm going this rout because I get to see exactly what the finished part will look like while it's still in plaster form. Plus, if I ever nose the sub into something that's tougher than the fiberglass, I still have the mold to make a new nose for it.
For the aft 'Main Ballast Tank," I plan on skipping the plug process since the shape is simpler. The shape is a simple parabolic curve with a radius of 18 inches on the sub end and a fixed point at the aft end. I've done molds of this sort of shape before and the easiest way I've found is to lay up a sorts sand mold using a blade cut to the parabolic shape mounted on a sweeping arm. I add a little masonry mortar to the moist sand and it stays in place. After the sand is formed to shape I use drywall compound to give it a smother surface and lock in any loose grains of sand. Paint it, lay on some wax and mold release, and then lay up the fiber glass part. Of course the sand mold is not easy to store so after I'm done making the part I want, it gets the hammer treatment and turns into fill behind the shop. My "Flinstone" method of molding, but it works and it's cheep. I molded a 16 foot diameter bowl shape this way once.
Dan H.
Captain Nemo wrote:
Well, I'm working with epoxy and glass right now, and it's still too wet to go to the next step, so here's a few things I think I know about this sort of thing, in case anybody is interested. Let's see...where to start? Some guys use either styrofoam or (even better) extruded polystyrene foam (pink or blue insulation from the hardware store) as the basis for a "plug" from which to make a fiberglass fairing shell from. This is a good way to go, but there's a few things to know about it. 1. Bondo will "eat" (dissolve) these kinds of foams. Bondo and some fiber glass resins contain chemicals like styrene monomer which dissolves styrofoam and polystyrene foam. If you make a shaping error and need to fill a ding in this kind of foam, don't use Bondo. Instead, scuff it with coarse sandpaper and pour in polyurethane foam. It will bond to the substrate without dissolving it. A little goes a long way: "surfboard foam" starts as a liquid and expands. Once you fill the ding, sand it as you would any other foam. 2. Fiberglass resin will also eat styrofoam and polystyrene foam (but not polyurethane foam). To protect styrene-based foam from being dissolved, coat it with epoxy. Epoxy doesn't heat up like resin does, so it won't melt the foam. It doesn't dissolve foam, either, because it doesn't contain styrene monomer. But it is expensive, whether you buy the amber stuff that comes in two tubes, or get it in gallon or larger cans, the price is high in comparison to fiberglass resin. Worth it if the project requires it, though. 3. Generally, curing time is a factor in determining strength. Slow curing epoxy is a lot stronger than the fast curing stuff. If you want to put a good strong protective seal coat on styrofoam or polystyrene foam, use the slow-curing epoxy. 4. Once you have the foam sealed, you can cover it in fiberglass. Scuff the epoxy with coarse sandpaper, and apply the glass. Then, sand smooth, apply your finish, and you've pretty much got a plug from which a female mold can be made. 5. If you just want a one-off fairing from this process (and don't want to make a mold) coat the ourside of the foam plug with several layers of fiberglass cloth for strength. Sand the exterior and get it as smooth as you want it to be. Then, dissolve out the foam core inside with a foam-eating solvent. What works for me is 100 octane low-lead av gasoline (airplane fuel, available at the local airport). It will melt the foam away; doesn't leave a bad smell when dry; cleans up easily with soap and water; and is relatively inexpensive as solvents go. 6. Once the foam is gone, if you want to strengthen the fairing, apply some fiberglass matting to the inside surfaces to thicken it up a bit. OK, I gotta get back to work. Any body want to add to this thread on working with 'glass? Pat