I've used Aqua Coat for several years and even wrote a review of it that appeared in American Lutherie. There is no need to "stay away from this stuff". You simply can't apply it directly to the koa wood. We used to apply naphtha based fillers directly to bare wood, but that is not usually done with water based fillers. Many, if not most, water base finishes, as well as Aqua Coat will give a greenish cast to the blacks in koa.. It is basically the only wood I work with, so I'm pretty experienced with the effects of fillers. Just apply your shellac or nitro wash coats before you use the filler. Do a decent job of keeping the 2 or 3 coats of filler level. No need to sand between coats. When you think you are done, sand the filler back and go on with your nitro or shellac.You won't have any trouble, even in the sanded through spots. If however you sand through and then apply a water base finish, you probably will get some greening. When the water base filler is used on the bare wood, the solvent base finishes can't wet the wood, which will give you a blotchy effect in any wood.