With the paint things are always tricky.
Growing up painting cars with my dad there are so many tricks and tips I could give you.
First off, when doing things like this, especially with metal and bondo, straight car paint is the way to go, hands down, it's the strongest and specifically made to go over multiple types of materials.
Your primer or base coat is always key and I always use a primer filler for cars, I very much do not recommend rustoleum products for cosplay, especially for what you're building. All these 2x coating is one coat or more are absurdly thick and can run way to easily. Sticking with single layer and very light coats is always best! Do not fret about how many coats you need to make, you should be making at least 3 anyway to do it properly, plus you can add more coats to certain areas to change the color depth and give things rounded hues that way too.
Automotive paint is also designed to dry and cure much faster! You color one day, you clear the next 3 layers each, boom job is done.
Please always use enamel paint, car paint is always enamel, you can't ever mix enamel with lacquer or acrylic! Which ever you start with as your primer is what you have to keep painting with, or the newest layer will eat right thru the lower one!! this is a horrible thing to go thru. Also remember that if you want to air brush things, especially with model paint, those are almost always enamel so that's another reason to stick with that type.
Now, for clear coats, I highly suggest using semi gloss or satin finish, you don't want gloss as it's always way to shiny, especially for a armor suit look. Gloss is meant for brand new off the assembly line cars. Also clear can run very very easily and you should always wet sand the final layer and then buff it smooth. I mean that's what you'd do on a car anyway. In this case I would highly suggest finishing with car clear coat if you can get it. Also 3 layers minimum. Car clear coat is way stronger than anything rustoleum offers and is designed to hold up to the harsh weather environments.
If you ever need special paints or colors made and in a spray can form, I have access to a place locally that can make any color spray paint and finish you can need and I can ship it too ya, all they make is car paint, and they always come thru for me so just let me know if you need anything!
Also, I'm sure you know most of this already I'm hoping that this info helps others too!
Keep up the awesome work!!!!