Like a foam, or a plaster, or a... imaginary magic space filler... thing.

[Paul Rhoney]

I have a project that is not actually guitar-related, but I thought you guys might still have some suggestions, being the rad bunch that you are.

Say you have a negative space, of a particularly odd shape, and you need to fill it. Not just fill it, but sort of overfill it, so that you can carve and sand back the excess to your desired shape. Anything pourable is out of the question, because you can't build walls around it. Wood or blocks of foam won't work either, the surface is really uneven. The ideal material would probably be modeling clay, but that's expensive, and besides requiring a lot of skill to lay up, you need a warm room and you don't have one. So what do you use?

My thought was plaster. Cheap, easy to work with, just glob a bunch of the crap on there, wait for it to harden and then carve it back. But I've never worked with plaster before, is it more complicated than that?

I'll tell you guys what I'm working on, it's a motorcycle that I'm making custom body work for. Ultimately the body work will be made of fiberglass or carbon fiber, but I have to first create the positive space shapes I want to duplicate, and they have to fit the complexities of the bike frame and some other components. Custom motorcycle builders do this kind of thing all the time, but they use blocks of foam, which won't work for me. So I'm at a loss, and am turning to y'all for help.

And for the record I'm trying to get back to making guitars too, but in the meantime, I wanna zoom around on my go machine.

[Chris Reed]

Theres a post in this thread which has useful links - they're making model sailplanes, but same technique. Plaster seems to work for small production runs.

https://www.rcgroups.com/forums/showthr ... te-Gliders

[Bryan Bear]

I'm not sure what the area looks like so this may be too messy but you should consider great stuff spray foam. If you can figure out a reversible way to protect the area being sprayed and allow the foam to release, this might be a good option. When it sprays out it isn't runny but not quite firm either. As it cures, it expands and fills in the shape of its container and pushes out of any openings. Its cheap, buy a can and spray in something to see if it might fit your needs.

[Brian Evans]

Most custom body builders use foam, either hard foam glued up, or spray foam insulation of a good density. I used to use a commercial two part foam to make race car seat inserts, two quart sized cans would yield maybe 10 cubic feet of quite dense foam. One trick is to use spray foam inside of plastic bags inside of whatever you are foaming, keeps the mess contained. If you are fiberglassing over foam, some resins eat the foam and some work well with the foam. I think epoxy is good, and polyester is bad.

[David King]

Most of the ready mix and 2 part foams are urethane which is impervious to styrene resin so fiberglass away...
Light weight carvable plaster can be formulated at home with just the addition of phenolic microbaloons available at any hobby shop. Wear a dustmask to avoid silicosis while carving and sanding.

[Paul Rhoney]

Spray foam seems possible, though potentially messy. My thought was to wrap practically the entire bike in cellophane (cling wrap), and then I could spray the foam on top. If I'm getting the Great Stuff ready to spray cans, which one do I get to give me the most carving density to work with?

David, do you mean just add the microballoons to regular old Home Depot big-bag-o-plaster? Would that make a less dense, but easier to carve plaster formulation?

[David King]

That should do it. Plaster is pretty easy to carve with rasps or surforms. You can chisel it away too especially when it's still hardening and warm to the touch. Plaster actually expends slightly as it hardens, something you may want to keep in mind. I just found this cool foam gun to make your own aircrete if you really want to get into this. You add some sort of foaming agent and the gun pumps your plaster slurry while your air compressor blasts air into it. The stuff has pretty amazing refractory properties aside from the light weight.
https://www.etsy.com/shop/LittlepigStor ... _shop_more

I wish plaster were cheap, Knez usually has the best prices, you might also want to experiment with "hot mud" drywall compound which is softer and slower to set than plaster and comes in bags at HomeDepot in 90 minute set or faster setting times if you so desire. You can keep working the surface as the material sets up and end up with an incredibly smooth glossy surface (just like plaster).

[Paul Rhoney]

So the Great Stuff spray foam seems like the perfect thing. I covered parts of the bike with cling wrap, and sprayed the Great Stuff on there. The next day it's ready to carve. But boy, is it messy! It's raining and without a dry shop to do this in, the mess is too much. I'll have to get back to it on a dryer day.

[Ed Minch]

I know I am a couple months late, but be careful of the "great stuff" type single part foams. They are moisture cure, and if you fill up a space bigger than a softball or so the center may never harden - a real mess when you are trying to shape it. Spray the foam in an inch or two thick, wait 10-30 minutes, add more, etc. cure time depends in the RH of the air. We get it to kick by spraying water on it from a Windex-type bottle. If it gets to the point where you can touch it with your finger and it does not stick, you are ready.

It is also great for shipping odd sized stuff, say a plane sent in the mail. Put the object in a trash bag, squirt a bit of foam in the bottom of the box, wait a few minutes, push the bagged object into the foam, then add those 1-2 inch layers until the box is full, and you can cut off the top even with a serrated kitchen knife.

Ed

[Paul Rhoney]

Absolutely right Ed, I've been messing with it, and found this to be the case. Still, trying to make it work.

[Ed Minch]

Paul

If you use the can with the soda straw and the built-in trigger, you can;t stop and re-start, you have to use it all once you start. The next level is a small gun that screws to a slightly different can of the same stuff that allow you to squirt it when you want. If this interests you, contact me.

Ed

[Paul Rhoney]

Sounds useful Ed, consider me interested.

[Ed Minch]

Paul

PM sent

Ed