I've been a licensed auto body and paint tech for 32 years, restoring Mopars for almost 40 years. I'll share some tips on bare metal and proper ways to deal with it.
As for your sandblasted fenders, first off, keep your bare hands off of them. The sandblasted metal is somewhat porous now, and the oils on your skin will contaminate the metal potentially causing corrosion wherever you have touched.
When dealing with sheet metal, you need you consider it's surface. Brand new, or nice shiny sanded bare metal is too smooth for anything to stick to well. From the factory, they use a process sometimes called bonderizing. Basically a phosphoric acid wash that etches the surface. This etched surface provides the tooth needed for primers to adhere to. So, on shiny, smooth sheet metal, use a metal prep acid, following the instructions, to etch the metal surface before coating with epoxy primer.
However, you cannot put polyester filler over metal prepped bare metal. It will react. That is why filler over epoxy primer is the best choice.
DO NOT use metal prep on sand blasted metal, because it is already mechanically etched, and porous.
Self etch primers are a primer, that has some acid in it to help adhere to smooth metal. Self etch primers really should not be used on sand blasted metal, for the same reason.
I'm not really a fan of etch primers. Filler cannot be safely applied over top because of the acid in the etch primer. Etch primer has it's place, but it's more geared for sealing bare metal that doesn't need any repair.
Sandblasted metal provides excellent adhesion for primers. Epoxy primer is your friend.
Epoxy primer comes in many variations. Slower curing products are better. Choose one that you can afford, and avoid any spray bombs. Epoxy primer is catalyzed, and once cured, most are stable and compatible with all fillers, primers and paint being put over them. I won't recommend one brand over another, but SPI, PPG DP epoxy and others similar to that are ideal.
So, with your newly sandblasted fender, put on your latex gloves, sand it as well as possible with something like 120-180 paper to knock down the texture and help remove the sand residue. Blow it off well, and wash it down with a final wipe cleaning solvent, intended for paint prep. Follow the instructions for the final wipe. Spray it with a quality epoxy primer, and once cured apply your body filler. Block sand, repeat the cleaning process, and apply epoxy primer again, before applying any filler primer.
So in a nutshell:
Properly prepared-cleaned metal.
epoxy primer to seal the metal.
filler and primer work over epoxy primer.
Cleanliness is ultra important throughout.
Always use products that are compatible.
Read and follow instructions.
This is a very simplified description. Auto body work is a complicated process, that requires skill, understanding of products, patience.
PS, as soon as you mentioned having a fender sandblasted, I got scared. Body panels can be made into scrap metal very quickly by someone with a sandblaster.
Every step from paint and rust removal, to polishing the final paint requires experience, and care. If you are new to auto body repair, I would suggest starting on less desirable-valuable parts to learn the skills.