Glass bubbles & epoxy as a core?

I am building a hillclimb / roadcourse car with composite panels and I have begun the fenders and quarter panels without using a mold. I’ve been using glass bubbles and epoxy to smooth things out. I got thinking that some parts of the panels aren’t as rigid as I would like, and that perhaps I should lay some thin carbon layers over the fill, making a thin sandwich in some places and then beautify over that. I read that glass bubble filler wasn’t very strong. My question is “should I strengthen the panels with a lamination on top of the filler, or just lay up more fabric on the back sides?” These panels aren’t structural but need only to resist the weight of someone in the paddock sitting on the flare etc. I’d like to take advantage of sandwich reinforcement,unless it’s a bad idea. What do you think?

I thought that I should explain things a little better. My reason for not making molds for the fenders, and just doing a wet lay up over top, was firstly, lack of knowledge, and also lack of time. I feared I would die of old age before I could finish the car, if I took the time to make molds. Also I’m half ways competent at smoothing out the bodywork, but inexperienced at making molds (especially 2 part molds).

As you can see in the images, the doors were done with a mold, but they were a simpler shape, and were done after the fenders. I’ll repeat my question. Do you think I should lay up carbon fabric over top of the glass bubble filler in order to increase the rigidity? thanks

In my experience epoxy and a filler make a good adhesive. I don’t think you need material over the filler. It’s a lot stronger than Bondo or polyester or vinyl ester resin mixed with cabosil.

Thank you Ro Yale. For some reason I thought that the glass bubbles and epoxy might crumble if stressed. The quarter panel did start feeling a lot more rigid with only a thin skim of filler applied. I made the panels thin, as I’m aiming to keep the car under 1500 lbs.