Hows it going guys? I'm currently prepping an 85 Corvette for a coat of epoxy primer + BC/CC. I've never messed with fiberglass before, so I'm honestly not sure if I should be sanding all the way down to the fiberglass (I notice a few layers between the glass and paint, see pics). Can anyone enlighten me as to what these different layers are, and which I should be sanding down to? I know I'll have to go all the way down to repair the damages, but what about the rest of it? It appears to go Paint > white layer (primer?) > dark blue layer > gray layer > fiberglass.

Also I noticed that in the front (where there appears to have been a small square cut out previously), the material & layers are different, and there's a yellowish material (pics). I also see it on the flexible bumper through some paint cracks (pics). These layers appear to go Paint > dark blue layer > yellow material.

Also, this car has a salvage title when I bought it, so I have no idea if previous work/paint was done on it.

First, welcome and here comes some info..... Technically, no Corvette made after 1973 is made of fiberglass, well, at least not the kind of fiberglass most people know. Stuff after that date went from "open" molded fiberglass to SMC which stands for Sheet Molded Compound. SMC is made in pressuremolds with a "mold release" cut right into the mix. This allows the part to cleanly release from the mold maker with no additional mold release butter or agents as we see in open layup molding. What this means to you.... you cannot use regular fiberglass repair products on this fiberglass. Evercoat makes several resins and other compatible products that will say right on the can SMC Compatible. Now a lot of the surfacing fillers will indeed say they can be used with SMC. You need to see that on ANY product that you will use for repair on this fiberglass. If you use any regular non SMC the stuff will literally start peeling off within weeks.
Okay, now as to what you've got in the way of paint.... Corvettes were usually hit with a surfacer/primer (some people mistake that for a gel coat, and NO it is not) of what color I am not sure. I've heard dark gray, black, etc. From there they would have went into their paint system. If you are seeing some yellow up in the nose area I would think that might have been repair work done with long or short haired fiberglass fillers. An example of something like that is USCs Durgalas (although it is more greenish in color).

Don't know what your budget is like on the car but if you've got the bucks I would have the surfaces blasted with deformable plastic media. It is safer for the surface and would keep you from "nicking" into that raw SMC. SMC can powder up and get very unstable as you sand "into" it.
Now one of the guys on here recently did have his Vette body soda blasted and his paint job does appear stable. There is real pros and cons to the soda blasting process and a cheap set-up can even be had from Harbor Freight (don't kill me guys I'm just the messenger). You can read about his 76 vette restoration experience here....
viewtopic.php?f=13&t=24221

So can you just sand this thing down?.... well, sure but it will be a learning curve of carefully sanding down to that SMC surface. Again, over sanding that surface will make for a ton of work filling, guide coating, blocking, etc., to get a wave free profile up and down that curvy body.
Hope this is enough to get you started.....

I was looking through some of my old links.... this is a fairly good read about the evolution of the fiberglass process with Corvettes at the beginning and some talk about their paint systems near the end....
http://www.superchevy.com/how-to/vemp-1 ... materials/