I would use 300 over the 900. 900 is bitumen based, the 300 active anti corrosive agents in it. And I would power wash everything clean before application. I made that transition. It took a lot of time to get things clean.
Use 300 on exposed areas, 700 on internal frame areas and unexposed area. You can search my posts I posted photos when I did it.
No you cannot. You need a pressurized cup gun. It’s too thick for an oil/lanolin based gun. You must also clean the equipment with solvent after you’re done