Marvelux is not high on my list of favorite commercial fluxes either as it absorbs moisture from the atmosphere and encourages rusting on my casting gear. Following the use of Marvelux, I have to wash and dry my stiring tools, ladle, and keep an overnight watch for moisture collecting on my casting furnace.
I currently use flux supplied by Magma Engineering (www.magmaengr.com) which has none of the problems associated with Marvelux. It's a white, solid, waxy flux which will smoke a bit but used outdoors or in conjunction with a fan, IMO, is not objectionable. You may request a free sample online and try some yourself. That's what I did and it convinced me that this is a better product to use while casting. The remainder of my Marvelux will be saved for smelting only and when that is gone, won't be replaced.
On the other hand, a lot of the guys here use a homemade flux and seem quite satisfied with it. You should be able to find out what they use by searching this site.
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