How do I fix a rusting panel on my front door?

Unsure what you mean by side panel. Are you talking about the side light panel or the actual edge of the door?

Remove the rust with a wire brush. If it's not rusted through, you should be able to prime and paint.

Its not rusted through to back, inside of house, but totally through first layer outside thanks

Go to your local home improvement store and find out what you should use to fill in the first layer, or will sanding it smooth be enough. Take a picture with you. Paints made for metal, like Rustoleum will work, they come in different colors, finishes and you can choose the texture, color, etc. that you want. Spray it and you won't get brush stokes And use thin coats and you shouldn't get runs in the paint. Follow the directions on the can for prep work and time between coats. It may be easier if you take the door down. You can do this, Kathy, and have your door looking new again.

Bondo