I've used Duram's water putty for my other hobbies and tightbond as well......although I haven't used in conjunction as you are proposing, I don't think it would mix well and the glue's liquid content might not be enough to activate the putty to adhere to itself.....in might just clump together. Both are sand-able but again, not sure how well they would work together like this. If you are worried about the safety aspect, go the glue way only. Instead of resin and bondo, use glue and cloth. Same method but different material. the waterproof glue is very thick and hard to "Paint" The original formula of Tightbond paints a bit better. Styart w/ the outside first and paint in section and allow to dry before moving on to the next. I would say 1/4 each time till done. Do 2 applications and then if you want to seal it, use 1 of the waterproof flavors but save that for last after you've smoothed it out. You'll see why shortly. For the interior, instead of fiberglass and resin, try glue and cloth strips instead. An old t-shirt will do nicely. you will need to water down the glue a bit (don't use the waterproof flavor for this) and soak each strip and layer the inside of the helmet. 2 applications for this should harden up the helmet enough for you to finish the outside. Instead of bondo people use joint compound for the smoothing out process on some of their armor, no reason you can't use here as well. Duram's is just the same as joint compound but w/ a higher PSI rating. Just makes sanding a bit harder but your overall piece will be stronger for it. I would mix up a batch but make it thicker than normal otherwise it will run off. Here is why you should wait till after to put a coat of waterproof glue on. You can't put anything on top of the waterproof coating. It won't form a good bond. You would have to sand and scuff the surface to get a good bond which defeats the purpose of sealing it. After you get you helmet sanded down the way you want, now you can seal it.
