Bondo resin is essentially the absolute low end of the product spectrum. While I'm sure it has served many people well, it's still a low end product. It shrinks, stinks, and you have to mix the heck out of it to get it to cure uniformly. Shelf life is also limited. it's also not very strong, in the grand scheme of things. While I wouldn't classify it as brittle, compared to other pro-grade products, it's junk.
I personally recommend the West Systems line, but it's price tag is not for the squeamish. But as usual, you get exactly what you pay for. West systems has a much milder odor than the entirely toxic 3m/bondo stuff, has a more forgiving and easy to manage mix ratio, and cures a very light amber. It bonds to itself and ANYTHING else very strongly. This is the stuff they build boat hulls out of.
To sirciarly, if you find mixing by volume difficult, I recommend picking up some clear cups and just putting them side by side on your work bench prior to pouring. It may quite possibly be the most simple way in the world to measure out equal portions of resin. Most smooth-on products are FAIRLY forgiving with their mix ratio, so if you are off by a little, it will still cure.
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In direct answer to your question, there are many brands far superior to Bondo/3m. I would even go so far as to say ANY other brand is better. No, it is not marine grade. I would rate it as an experimental/hobbyist grade.
