I picked a model that would be pretty challenging (all curves) but not ridiculously complex. There are limits as to how delicate your details can be and still come through on the "printed" core - but adding details would be easy later on in the process. It is a great way to obtain the overall structure of a piece.
Even though it's dramatically less expensive than having a rapid prototype printed (3D print), it's still not terribly cheap. I got 40% off on my laser cutting and it still cost $115 for the service. You *could* cut out all of the pieces yourself, but I'm in favor of retaining all the sanity I can - lol. Once everything is added up, you could be looking at a few hundred dollars once you add in filler, primer, paint, sandpaper, glue, etc. The price will be dependent on the size and complexity of your project (you pay by the linear inch of cut for the laser cutting service as well as materials) and whether you decide to go for less expensive materials (paints, primers, etc).
Cardboard "core" sculpture
Body Filler (I used the Evercoat brand as it's the smoothest I found)
Plastic spatulas (some way to spread and mix the body filler)
Glue (I used yellow glue because it allows for some movement as you place layers of cardboard)
Weights (some way to clamp the layers as they dry)
A few pins (used to align the layers with each other)
Sandpaper (I didn't use very much, really - maybe a sheet of each - 80, 150, 220, 320, 600 grits)
Primer (a good quality FILLER primer - I used a catalyzed filler primer by Keystone - #8882)
Paint (if you want it - could be rattle can paint - or something more exotic)
Sureform or Rasps (used to shape the rough filler)
Misc (this is the catch-all category - tape, drop-cloths, etc)
Step 1: Process Your Model
Once you import your model into 123D Make, the first thing you'll probably want to do is to choose the material and thickness that you intend to use. Knowing the thickness of the material you plan to use is *critical* in order to get the best (non-distorted) results. Next, you'll want to set your scale - overall dimensions of your final model. Next choose the best layer orientation for your model - one that will minimize or eliminate unconnected parts or leave you with small or unsupported structures. When you are happy with your results, export your templates and either cut them out yourself, or have a laser-cutting service do it for you.
Step 2: Assemble the Core
Say you have four layers - A, B, C, and D. Each layer will have four holes - roughly in the 3, 6, 9, and 12 o-clock positions.
Layer A is on the bottom - it may only have two holes - but let's say it has four. When you place layer B on top of layer A, only two holes will align correctly with the two of the holes in layer B. Let's say it's the holes at the 12 and 6 o-clock position. Spread glue on the bottom of layer B, and using pins at the 12 and 6 o-clock positions, align the layers and press them together.
Now, when placing layer C on top of layer B, the alternating set of holes will align - the 3 and 9 o-clock positions will be your reference points.
When you place layer D, you will again use the 12 and 6 o-clock positions, etc.
This should be a bit clearer when you have your parts in front of you, but it escaped me until after I had several layers glued up - so it's worth noting.
Between each glue-up, I would put a board topped by a 5-lb weight on top of the stack to hold it all tightly together until that layer dried (about 20 minutes between layers). I don't recommend building separate stacks and gluing them together - they may be hard to align, or have a slightly different warp to them and not glue up tightly.
The final core is pretty tough - but I wanted to make it a lot more robust and make it easier for the body filler to bond to it, so I coated everything with clear urethane.
Step 3: Filling and Smoothing
I tried a few types of filler and found that the Evercoat brand was the smoothest and easiest to work with. BTW - All of your filler should be mixed and used with plenty of air circulation - outside if possible - and it'd be a good idea to wear an organic vapor respirator. You don't HAVE to, but I prefer to NOT smell polyester resin for the rest of the day - it's one of those smells that seems to get into your nose and lingers even when you're nowhere near the stuff.
Early on, I considered using spray foam to fill the voids - and I think that's a possible solution - but I wanted something that was going to be *very* smooth and *tough* - so I went with body filler. I'll try foam next time ;)
The first few layers were just for overall shape - the whole process is one of refine, refine, refine - don't expect it all to be smooth in one step and you won't get frustrated. Take your time, keep moving around the model, don't get focused on one area. Eventually, you'll only have small imperfections to fill - and that's the job of filler primer ;)
Step 4: Priming and Final Smoothing
Again, it's all about refine, refine, refine. I sprayed something like six coats of primer (spray two, cure, wet sand, spray two, cure, wet sand, etc)
In regards to sandpaper: Most of the sanding I did was with 1"x1" squares. I cut 1"x3" long strips, then fold them in thirds. This gives you three fresh surfaces to work with. Always let the sandpaper do the work, don't use too much pressure. You want the paper to glide over the high spots and take them down - using too much pressure can scar the surface, and will force the paper INTO the low areas which pretty much defeats the purpose of sanding. A good way to judge pressure is to rub your finger quickly along the inside of your wrist using the pressure you think is good - if you don't generate a lot of heat, that's about right.
Once you think you're all smooth, it's time to focus on high spots, rough spots, and pits. I use semi-cured primer as a filler for the pits I find - it works better than glazing compound. Sit by a strong source of light and carefully pick out the problem areas - I mark them with a pencil and a visual code that lets me know what the problem with that area is.
Step 5: Painting
A three-stage finish is one that consists of a base coat, metallic coat, and a clear coat. The finish I chose was a pearl white from a 2002 Cadillac. I don't have pictures of the finishing process - mostly because it's a fairly frantic process and I was working under less than optimal conditions (windy, cold) but overall I'm pretty happy with the finish.
At this point, I'm probably going to pull molds off this model and do some castings - maybe some cold-cast bronze, resin, ceramics ... even wax(?) Sounds like another Instructable to me :)
So, would I do it again? I think so. It's a lot of work, but I think that next time I'll be able to streamline the process and probably do it in about half the time. It seems like you always get better results the second time around :)
Thanks for your time! I hope you found it interesting ... and maybe even ... inspiring :)