For good sanding qualities, you need a surfacer.
Polyester, 2k urethane, or hybrid are your material choices in modern surfacers.

For corrosion resistance, epoxy is a very good bet direct to metal which is why many of us start with an epoxy and then apply surfacer of some kind on top of it.

On the GTX, SPI epoxy direct to metal, then SPI 2k urethane surfacer for blocking purposes.
They have a standard and a high build, I used both but preferred the standard/it sanded nicer for me.
https://www.southernpolyurethanes.com/
They treated me right. Shipping was included on everything + the tech line is the owner's cell phone.
They also have their own forum like this one with a ton of information available.
I also used their clear coat.

Years ago used pre-Valspar Matrix epoxy + 2k urethane surfacer combination before with good results, but don't know if the product is the same now.

Next project I want to try a polyester surfacer after epoxy, as I think it will help me do the job faster.