Painting over bare metal without a quality etching primer first would be a waste of time. I'm not sure how rattle can primers would hold up since I've never used them. I use a catalyzed primer in a spray gun. The paint used on your truck is a base/clear system, the base color is only color and dries flat mixed with a stabilizer. The clear gives it the shine and is mixed with a reducer and hardener but you would need an air supply and spray gun. Ideally you would use a sand blaster to get down to nice clean metal removing ALL of the rust. If pitting is a result of cleaning the rust away you would use polyester glaze and block sand smooth followed by a primer then top coat with a catalyzed sealer preferably over the primer. The tailgate would be relatively easy just shoot it open. The other two spots you have a body crease line where I would use a round feather edge tape so you don't end up with a sharp paint line. You could get by with rolling the masking edge of the tape up achieving basically the same affect and shooting the whole lower section. Wouldn't have to paint the whole section only the primed area fanning/blending the paint out but clearing the whole section. Same could be done with the fender lips. The toughest one to have good results would be the fender since there isn't a real defined line there. One option would be to repair/paint the area to stop the rust and put fender flares on which you can get a whole set of for less than $300. After you remove the rust by blasting/grinding or whatever your going to need to feather the repair area so there isn't a noticeable ridge for lack of better words. An orbital sander works best for this but can be done by hand. 360 grit would be fine then use 400 on the primer. Problem is having the tools, mainly a clean/dry air source and spray gun. Red is your most expensive color, have seen pints go as much as over $140! Then you will need a stabilizer cuz paint is usually reduced 1:1, pints might be available dunno we buy by the gallon. Same thing with the clear not sure if that's sold in quart form cuz again, we buy by the gallon. I would probably recommend using a high speed type clear since you likely won't have a real dust free environment to spray in ( the clear doesn't stay "open" as long and doesn't run as easy...would wait for a warm day too if you don't have a heated shop)this also eliminates the need for a clear coat reducer. Take the time to cover rest of truck from overspray it's a dam to remove trust me. You can see why it's sometimes not so quite "cost effective" to DIY sometimes Good Luck! Oh and make sure and clean the areas well with a paint prep/wax remover, any type of petroleum type contaminant will cause fisheyes ie. wax/armor all/ tire dressing and that greasy Big Mac on your fingers etc. Good idea to have a fisheye eliminator on hand. Tac rag too to remove dust.