I've been wondering, is there a way to gratin food (as in cheese gratin) in a gas oven without a broiler?  I'm used to cook using my parents' big electric oven, which had coils in the upper and lower part, so I was able to heat from below as well as gratin from the top.  Now I only have access to a gas oven without a broiler, but I still want to get those nice gratin crisp cheese on lasagnas and escondidinhos (a traditional dish from the north of Brazil).   
I don't know about the brand, but flours which are sold for health purposes are often whole grain. If it has the bran milled together with the corn, there is no way around the grittiness.   I don't know if something like "teff cake flour" exists, as it is probably the opposite of what teff eaters want. You could try calling the producers of your brand and asking. Else see if cycling through brands helps. But for such a niche product, it might be that all brands do whole grain only.  