This got me intrigued - what flora, exactly, are you cultivating on your hands (the area that would normally be exposed to hand sanitizer) ?
Or did you entertain the thought of ingesting hand sanitizer, per certain federal politician's recommendation?
How exactly does hand sanitizer effect your gut flora?
Bondo is lighter than epoxy w/o fillers.
It is also fairly brittle.
Many use bondo in rocketry, just not for structural reinforcement. Bondo works great as a spiral filler for paper tubes.
Cars don't routinely bounce off the ground at 20+m/s during landings, or bounce in the trunks during transportation.
So the comparison is a bit of a stretch.
Sanitizer goes on hands, hands go on food, food goes down the tube with trace amounts of an ingredient that acts as a broad-spectrum poison against bacteria, even the ones that live in your gut and help you digest food. Alcohol-based sanitizer contained denaturing agents which are directly poisonous to human beings and just about everything else (It ain't Everclear) Alcohol, depending on concentration also selectively chooses its victims in the microscopic world. Acetobacter loves getting wasted, but a high enough concentration will kill it.
There are volumes of data freely available on the immune system online, and I've already done enough derailing to this thread. If you are truly interested, there are scatterings of journal articles, clinical reports, theories, and of course the usual internet BS available on the subject, all with different opinions on the specifics.
Suffice to say, when I see someone slathering their hands with sanitizer prior to diving into a McSandwich, that makes me more nervous than the germs they are trying to kill. I like talking about this stuff, but this isn't really the place. I think I sufficiently answered your query on my reasoning, and the above is just that, my opinion based on my own research and understanding and direct experience.
On with the show.
I've used Bondo and I kinda like it for ease of use and light weight. There are several products that contain additions or different formulations that I think would be just fine in rocketry, but I don't have much direct experience with them. Would kinda like to try kitty hair and see how it holds up.
In any case, I use bondo for finishing and filling, not as a structural element. And with TTW fins I'd hardly consider fillets a structural element. The attachy parts are down below.