>I'll try the Elmers glue you suggest next time.
OK, but just so nobody gets confused, that's Elmer's PROBOND POLYURETHANE GLUE; not the white stuff.
>Once it was roughed out, I used Plaster of Paris to develop the shape farther....I then switched to drywall compound. Bondo takes much more elbow grease to sand down.
Yep. BONDO's also far more expensive, and its fast cure time makes it harder to work with; not to mention the fact that it eats foam. I like this plaster / drywall compound method and will try it on the large parts I'll be making soon. Sounds much easier and more economical.
>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.
QUESTION: Any personal preferences for wax and mold release: any particular brand names or have you come up with any economical "homebrew expedients"?
>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.
With the "lost foam" method, you can make a one-off fairing, but the exterior finish-shaping will be a lot more difficult than your method. And if one is planning on making a female mold from the plug anyway, making it out of fiberglass is less expensive than making it out of, say, polyurethane rubber. I think two deciding factors are size and complexity: If the object's surface is fairly large, smooth, and doesn't contain a lot of fine three dimensional detail, the "mud method" sounds like the way to go. (No negative implication indended: just a simple name based on the fact that, over here, we call drywall compound "mud".)
>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.
Very true. And the mold enables us to make more than one exact copy of the part: like, say, two matching fairings, one for each side of the sub; or maybe four "quarter sections" that will eventually be assembled to produce a single hull fairing or the like. And who knows; people might like your fairing and want to buy a copy. With a 'glass mold, you're in business; and you'll get a lot more "pulls" off a 'glass mold than a rubber one. Again, the size and surface detail of the finished object are factors to consider in deciding what kind of mold to make, but either way, taking the time to make a mold can be a good way to go.
>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.
Now that's a handy tip I think I'll add to my notebook. Never can tell when it might prove useful, expecially for us guys working in our garage without sophisticated equipment. I like "Flintstone" stuff, probably because that's all I've ever had to work with. I never cease to be amazed by what people can accomplish with a little ingenuity. Thanks for the tips, Dan!
Pat