Bondo is a decent filler & if you use it in small batches, get the right mix with the hardener it works well. Getting it applied smoothly has to do with how fast you work with it & get it applied. But there are other brands that would work just as well.
Lacquer spot putty works well but need to be wary due to being lacquer based & not enamel it will interact differently with most primers. This works well for small spots & areas but not good for filling larger areas.
Another low cost method that has worked well for me is to use Spackle & add Elmers white glue to it. Both being water based they mix well. I usually mix till I have a smooth consistency, don't really have a ratio per say. By adding the Elmers white glue to the Spackle this acts like a hardener & firms up the Spackle when it dries. Very easy to sand & isn't as powdery, I have used this on sorts of things from helmets to armor & it has held up very well.
You can try Durham's Water Putty but I have had mixed results with this cause it is powder & you have to mix it, sometimes doesn't come out the same. But if you work with it & come up with proper ratios for the mix it can work. Have also added the Elmers glue to this as an extra binder that seemed to help when I have used it.
I have found that not one thing is good for everything so sometime you have to use different filler depending on what you are doing.