These guys already gave you some good advice, so I will be repeating a bit but also have some to add. Use the longest block you can comfortably use in the area you are working. It will be easiest to get things straight on a decent sized area with a course grit, i use 36 grit if regular filler, not a finishing type. worry about switching to a finer grit and taking out scratches after you have your plastic filler nice and straight by either going over with 80 and 180 or putting a nice neat and tight skim coat over the whole area and sanding again with a finer grit. When you are working with filler, you want to fill aways past your actual low area (Its best to get it worked out primer with hammer and dolly work as well as knock down any high areas). Sand in an x pattern completely from one end to the other end of you filler spot or as far as you can until it feathers out on the edges. Use your hand and feel what things are feeling like. Once its feathered out well if you don't have high areas that need more sanding, stop, and if you still have low areas, you need another layer of filler. I like to start working the filler when its still a little soft and sandings easier even if it loads the paper a little. But if it is tearing out instead of feathering, you should let it sit longer before you continue sanding.
The spray can filler not real good. Will be a lacquer primer, which shrinks a lot. Can shrink and have sand scratches show up later or leave rings around filler areas. Its best to let it sit for a least a couple days before final sanding for paint and allow it to shrink. Years ago its all their was to work with, but today epoxy and urethane primers are much better. Epoxy is the only primer that is not somewhat pourous and has very good adhesion to metal and will provide good corrosion protection over sanded and clean metal. They normally don't sand real nicely or have a lot of fill, so many will apply epoxy primer first , do filler work over the epoxy, re epoxy when filler is all blocked straight to seal and cover spots through to metal, and apply a high build urethane primer for it fill for blocksanding and scratches, and its sandability. That lacquer primer will provide little in the way of protection over bare metal, and also isn't best for adhesion to it. 2k primers are superior but it is very important to protect yourself when spraying to avoid getting in lungs and nervous system and have good ventilation. And also you need spray equiptment as they can't be put in a can and stored on a shelf, as they have a relatively short pot life when mixed.