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When you finish your pull and have a piece made from smooth cast 320, roto, etc. how well do the parts take to sanding? I know ideally you should aim for a perfect buck before molding, but is ok to try to continue smoothing on a casted part?

as i have never used smooth cast products i'll try and answer the second part of your question. It would depend on how much sanding you have to do and what the reason is. if it because of a flaw from the molding process.. then yeah a bit of sanding is alright.. if its a flaw with the master then more work is needed on the master..

I agree with twistedxknights. If it's because of the master mold, then you should aim to have that mold sorted out.
As for Smooth-cast 320, it's a dense enough plastic resin to be sanded but I would suggest using a fine grit (probably anything #100 upwards) followed by buffing blocks. The thing with Smooth-Cast 320 is that it's meant to be pigmented when it's first used - it's pretty much impossible to paint directly, as the smallest, softest scratch will cause the paint to lift straight off (I've seen this with a variety of paints, too)...

Its not a good idea to mold a incomplete piece, but if you find with some issue in your cast ( i keep getting two noticable wrinkles on the left side of my helmet mold everytime )

It is sandable but it sure feels like it takes longer to sand compared to body filler.

I also concur with chantelle about painting a raw cast. I thought i would be clever to primer first then mold. End results: most paints did not work very well on the casts.

You can either primer it or roughen the surface with a fine grit sanding sponge

Well, my question was leaning more towards the pre-painting aspect of the piece but thanks guys.

I used Roto-65 and the stuff takes well to paint and sanding. It's actually easier to sand than the bondo imo, simply because I can get in the tight areas and sand by hand instead of relying on a mouse. I seem to get things accomplished quicker ^_^

There is no issue with major details or symmetry, everything is good to go. Just minor surface issues that, when working with bondo and it's pitting habbit, seem to take forever to fix - which actually adds more refining time. Sanding on Roto seems faster and gets me where I want to be in the long run without all of the hassle.

Thanks again for the advice guys.

I dont know if this is helpful or not, and since I havent started molding just yet I dont know this from experience. On several of Ithicas videos he talks about doing several "generations of molds". Gen 1 is from your master, as good as you can get it. Then he modifies and further refines, sands, and details his cast to perfection and makes a second generation mold. Gathering from this it sounds like the casts take well to modification and sanding, but I imagine this process can get expensive with the mold materials.

I'm using SmoothCast 321, and finding that sands pretty nicely with coarser grits and, after doing that, takes smoothing with finer grits pretty well, also. Like everyone else said, you should work your master as much as possible, but some things are not as easy to do with clay because it is so very sensitive, and sometimes you need something a bit less malleable to finish. That's where transition molding comes in (the "generations of molds" that hollywood mentioned). You make a mold, do a cast with a more forgiving material, finish the cast, then make another mold. Voila!! Now, if you do a good enough job with the original master, you may only need a little bit of work, and you may find it's not worth the trouble to do a second round of molding. For my project, I ended up having to do a good deal of shaping with the sandpaper, leveling out some uneven spots that I couldn't fix in the clay, and it's been turning out all right so far... Except for one small problem, which I'm hoping someone here can help me with...

After sanding, I'm getting a lot of little pinhole pits, likely caused by tiny air bubbles in the liquid plastic. Is there some way to minimize this problem during the slush casting process, or should I just resign myself to trying to fix it with body filler later?

As for paint, the 321 seems to be taking paint just fine, as long as you use a coat of primer first.

only thing i could suggest is to use small batches when your slush casting instead of one larger batch

SK: Some products release vapours in heat and humidity that cause that, and other times it can be caused by dust in the mould. Not sure what would have caused yours, though!

To help eliminate the tiny air bubbles your resin really should be degassed before the pour. If it is a fast cast resin that you wouldn't have time to degas before it kicks off, you can degas both parts (A & B) seperately then 'gently' mix them together with a stir stick before pouring into your mold. A couple other ways to help eliminate air bubbles in your casted parts... especially if you don't have a degassing station... is to be careful 'how' you pour your resin into the mold. A careful pour that doesn't allow a lot of bubbles to foam up will help eliminate bubbles in your casted part.

Another trick I use a lot was to dust the inside of the silicone mold with baby powder and then bang out all the excess so your left with a nice even LIGHT dusting. It helps the resin flow throughout the mold and helps eliminate air bubbles especially in detail areas.

I have also seen additives that are supposed to have that effect. They were simply called "degassing agent" (rough translation), so I'm afraid have no idea what they actually consist of, but they might be worth looking into.

thanks for this it really helps