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I am no beginner to scratch builds. That being said, I am thinking about scatch building a Tau Superheavy at some point in the future, possibly a manta.
I ALWAYS make my things to last forever, so simple cardboard is never an option. So far I have become pretty good at free-handing compound curves from blocks of wood, but this is a difficult and time consuming process.
Here is what I am thinking. I am going to make a light weight "skeleton" from plasticard, legos, a little styrofoam, and small tupperwear containers. This will give the basic shape. I will then cover the entire thing in Bondo, and palm sand it into shape. Then use primer it, and sand again.
Does this sound like a good idea, or will I just be wasting days of my time for something that will weigh a ton and break easily?

Another option for your lightweight frame is aluminum foil. a box or two is cheap, and when compacted (scrunched? smushed? It's hard to be eloquent with squishing foil balls I guess) it gives a great base for sculpting, and will form elaborate curves easily.

Also, most hardware stores sell "Great Stuff", an aerosol can of expanding sealant foam that could be slopped all over a sub frame and cut/sanded into shape once dry. With that though, you'd probably want to seal it before trying to paint/prime, or possibly putting a final "skin" of bondo over top to help ensure a smooth final surface.

I've actually used the expanding foam before. It does work quite well, just never thought to use it as filler in a model. Thanks for the idea.

Cuda, you mentioned you have experience using Great Stuff Expanding foam. Have you ever tried casting with it ?

I was tempted to try and make a Gargant in Expanding foam. If I go through all the work of making one, I wanted to be able to knock out a few of them.

Not something I have personally tried, but I have considered using clay to make the core, rounded parts of a model and then once it dries make silicon moulds of the clay and then cast it in (lighter) resin.

There´s a blog at Advanced Tau Tactica chronicling the creation of a very impressive scratch build Manta:

http://advancedtautactica.com/viewtopic.php?t=7604

He´s using ribs to define the outlines and building foam to fill the gaps.

That man is a god. The work he is doing is amzing, both in its quality and its speed. I see some room for simplification and improvements, but he was doing this blind. Thanks for the link.

you could try sheets of very maleable plastic or metal and heat it up. See how that works. Lots of work though.