yes, it is, but based on my experience here's i've done. . .
i did fiberglass foam helmets btw. . :lol in this method, fiberglass first the inside. .then cut the helmet's faceplate and the lower back section of the helmet. . .then seal it first, either with plastidip or PVA glue, i'm using acrylic emulsion to seal my parts. . .then after you have sealed (i think 2 for plastidip, 4-5 layers using PVA glue) the helmet, apply bondo regularly. . .then sand. .sand. .sand. . .take a break and a cup of coffee. .then sand again and again until you are satisfied with it. . .
note: it will make your foam helmet heavy. .mine's round 1.25Kg? :confused i think you can do it without fiberglassing the inside. . .just seal it then apply bondo. . .