We use the large popsicle sticks, you know the tongue depressers that doctors use/used?
Bondo will be much faster than gesso. Gesso you'll need to do many many layers to get a smooth finish, and you still won't have filled any larger divots.
Just bear in mind with bondo, you only have a limited time to get it on before it starts to cure. The more hardener you use, the faster it sets, and works the other way too.
Gesso would be the better option if you've got detailing on your armour, as bondo will be great for smoothing large areas but if you're trying to do it over a design it's just going to stuff everything up, whereas Gesso is much thinner and won't make your details disappear.