Do I need to sand this whole thing down!

Okay will do sorry about that monook...

What if the paint doesn't look solid? When I was sanding down the panels, I noticed a first grey which looked like rattle can primer, then yellow paint, and a red primer then sheet metal. My plan is to do the whole car glossy red.

No worry's It is just easier to follow your progress.

As long as the old paint is solid not flaking or loose. This has a good starting point. Things just have to be scuff (sanded) for the primer to hang on to.
Are you going solid red or will it have a metallic in it?
Solid is a lot easier for a first timer. But you are a long way from that yet.
Also you are from California and I do not know what products you can get
compared to here.

Darryl