POR-15 is a great product, but I have had it flake off of smooth surfaces before. I did used the Marine Clean and Metal Ready before applying. It also requires a top coat as it's not UV resistant.
As far as what it sticks to, the manufacturer would be a good initial source for recommendations. But I would try it out on something first and see how well it holds. Here is a link to their website, scroll down for a quick example of how to prep it and then look at the information sheets below that for more information you'll need to know.
>Link
(Note that the Metal Ready leaves a zinc coating, so I doubt that would stick to filler.)
As far as the comment on the rattle can, from what I understand, the formula or type of paint is not as good as what is sprayed. I have heard of paint shops filling individual cans though, but I haven't tried this option and I'm pretty sure it's expensive.
Over all, I understand not having funds, but if you take a step back and consider how much you've already invested in this, and the fact that it won't be as assessable as it is now, then doing the job right would be at the top of my list. Personally, I would probably try the POR-15 having prepped it and then do a scrape test on it before applying a top coat to it. Be careful of areas with smooth surfaces especially if they've had grease on them before as it's my understanding that metal can leach oils even thought the surface may be clean. I have heard of powder coating places heating parts to get rid of residual grease as well.