If your cold casting ALU specifically, the BEST thing to use is clear layup resin. Not to be confused with clear CASTING resin used for embedding flies in ice cubes 
Clear gel coat is good too. But it more "picky" becasue of all of the filler that is already in gel coat. Gel coat of any color, is mixed with cabosil or other fillers to "thicken it". When you put your Alu powder into it, there is twice the filler, or MORE depending on how much your using. All this does is ultimately make your mixture more brittle when cured. Not to mention, that you have more instances of catalization error with all of this filler, leading to wrinkling, and a plethroa of other hassles that could take a lot of time and material to experiment with. Take it from me, been there, done that. That's why my Jango Gaunt project has taken so long 
Used clear layup resin. Not amber. "CLEAR". It will be the most brilliant when you buff your casted piece. Any other color, or "pigment" just reduces the potential brilliance of your final piece.
There is of course, MUCH more to this that what's been said above. Some may get pretty good results just playing around with it. But for the best possible finish, there are many trick to be learned. What I mean by this is, most ppl don't care if they have to sand their finished pieces after casting before buffing and polishing. But this ruins the potential finish,permanently. It will still look like metal, but not like bright aluminum. It is of the utmost importance to keep the natural "skinning of your aluminum coating INTACT. That way when it's buffed after cure, it's as bright as it can be. Otherwise, if there is an imperfection, and you have to sand down past the skin coat,, it part will look like stainless steel or lead. It's just the nature of the process. The first priority is making the cleanest, smoothest mold you can.
This will all make more sense in a few days when I post the new Jango Gauntlets. Anyone can cold cast, but ther eis a huge learning curve to get the level of brilliancy that we seek in Jango parts as an example. Although, his gauntlets are the dullest part of his suit, so mine are probably going to be unnacceptable to most purists But nothing a little rougher steel whool won't take care of I suppose
Stay tuned !
BTW - When you ask questions like this, all you do is spark curiostiy? Whatcha up to dude?
FP
