how to tell if its bondo or primer ?

First go to the auto parts store and get a can of Bondo (just in case) then take a sharp instrument and apply a little pressure not enough to scratch the metal but enough to see if it gouges the bondo. Believe it or not 99% of cars have a slight patching done at the factory for smoothness. I see a lot of cars at the body shop stripped down and have bondo. If it is no problem as you can patch it easily if it gouges. you say you see no dents. they may be very slight.

Wash off the chemical strip and let it dry thouroughly before testing....

Most (if not all) auto manufacturers use bondo in the manufacturing process, before painting. It would not be unusual to have factory bondo filler under the paint. Use a magnet (there are special ones you can buy that will tell you how thick the plastic filler is) and unless there's rust under the filler I wouldn't worry about it too much.

Usually If it is primer someone had put a piece of sheet metal under it and welded it.. If it is bondo you will sand it down and find a pink like place.

Drill bit will tell all

Go slow with a sharp bit - if it's metal no biggie

If it is bondo it will drill right to the metal or lack there of

Put a magnet on it. If it sticks its metal and if it doesn't its bondo.

Go over it with a hammer. Bondo will crack or fall out.

Factory body filler where they put the two panels together.

Scince you already chemicaly altered it, you need to completely remove all of it. Otherwise, paint and primer will peel and bubble.

Some times at the factory--vehicles get dents. It is probable boddy puttie of some sorts. If not it is sheet metal.. You seen enought of that

mask of the section you choose to best leaving a minimum of three inches around the fix then spray a lite coat over you section enable this to tack off then practice yet another somewhat heavier coat depart for that coat to tack off then practice an extremely final coat the comparable simply by fact the final,depart approximately ten Min's between coats

go over these spots with a magnet.If it's just primer, the magnet will stick.