Preparing an old metal cabinet for painting
OK, so this is a classic prep-paint situation, on a metal thing which has alkyd (that's what its mom calls oil paint when it's in trouble) or possibly LPU paint.   Definitely not latex, so we dodged that bullet.  shudder

Latex paints are not for metal equipment like this.  They will fail very quickly. However alkyds are very stinky. If you can't paint somewhere vented, then use a waterborne alkyd with synthetic for-latex brushes... but those are new, weird and I don't trust them. 

Paint prep is a generally miserable job, and the picture postcard for a very basic rule about tools: Always use hand tools until you've achieved mastery of the hand tool. Only then, look at power tools, since you now have a well-caibrated sense of how you want to automate the job.  I own a ton of power prep tools. That particular piece isn't bad, and I would probably leave them in the cabinet and go at it with hand wirebrushes and sanding blocks.  I would go for a power wirebrush if there was determined rust there. 

But based on the fact that this isn't very rusted, I'm guessing it lives in a low-rust-risk location, so you don't really need SSPC-SP10 surface prep; any residual leftover rust probably won't break out.  You just need it aesthetically smooth.  

Prep is about

a) Removing and neutralizing rust.  For a low-risk situation like yours, I would use hand or power wire brushing and/or sanding, til very little remains, then hit it with a rust converter, which you paint on.
b) Smoothing the surface. That's what sanding is all about.  Dry-sand, or wet-sand if you're worried about lead.  Now, you also can use primer as a "filler", to fill in the low spots.  Primer -> sand -> primer -> sand -> primer is SOP around my shop, and they make high-build primers specifically for this purpose.  For deeper fills, you can use fairing (commonly called Bondo).
c) Roughening the surface. Fresh paint cannot stick to a glossy surface.
d) removing dust and dirt.  Mostly dust from the above lol.
e) removing contaminants.  Anything that'll mess up the paint - especially silicones, linseed oil, latex paint.  I prefer to take the primer I'm applying and use its own diluent (the stuff you thin it with, i.e. paint thinner for Rustoleum).
A double-cloth wipedown works best, cloth #1 is wet with thinner, and cloth #2 is clean and dry and picks up the contaminants.  Keep refreshing cloth #2, I prefer to graduate it to cloth #1. 

Amazon Prime

Now you prime that sucker.  You need to coordinate the primer to be compatible with the topcoat, both chemically and aesthetically.  

Decide whether brush or roller stipple is something you can live with.  If not, you'll need to skill up on rattle-can spray jobs. They are easy to mess up with runs etc. 

Rustoleum 7769 Rusty Metal Primer is the stuff you want. It performs WAY better than you'd expect for consumer tier stuff, it's available everywhere, and it won't kill you*. When I can't get a surface quite to SSPC-SP10, I use Rustoleum 7769, and then apply my industrial tier coatings.  You still need the unit quite dry, so drive humidity in the room down as much as you can. 

Despite Rustoleum's "Stops Rust" slogan, Rustoleum 7769 is the ONLY product that will do that, and only over fairly good prep. 

Rustoleum is an alkyd and is Very Stinky stuff. There is no way to avoid it. Water-based paints just won't perform on metal machinery like this.  And hardware stores are now trying to push you into  a "waterborne alkyd" - hissss! booooo! Avoid! 

Paint outdoors. Lay down and weight down a tarp first, and don't overspray anyone's car! 

You only need Rusto 7769 on the bare metal or formerly rusty areas.  If the old paint is solid, priming is not so urgent.  

Nothing wrong with hand brushing. You must use a natural bristle brush, not the synthetic brushes for latex.  Either buy a good'un and fastidiously clean it after every use, or I get cheapie "chip brushes" and just watch for bristles and pluck them out of the coat as it goes on. 

With spray, you'll need to multi-coat the first layer (if you try to get it in 1, it'll run).  Brush might not need a 2nd coat if you coat it well the first time. 

If you're after a smooth finish, you can re-sand after the prime. But if you blow through to bare metal, you'll have to re-prime at least that area. 

Now think about topcoat.

Rustoleum 7769 is too good not to use, but it leaves you with a deep brown.  If you want a light finish, prepare for 3 more coats. Normally I use Rustoleum 7780 for the next coat, but I gather you don't want to buy yet another can of something, so I just say use a 3rd coat of topcoat. There is no such thing as a 1-coat topcoat, because a) it's opacity isn't that good, and b) your application isn't that perfect.  Marketing lies!  

Since you don't need to Rusto everywhere, you might make your first topcoat just over the brown areas. 

Since this is equipment not a house, your topcoat should be an oil-based alkyd (i.e. not a "waterborne alkyd", yuck).  Rustoleum's topcoats (the alkyd "High Performance Protective Enamel" ones, anyway) are perfectly adequate.  On one hand, color choice is very limited; but on the other hand, a second can will match.  Sherwin Williams will custom mix, but their second can will be a bit off. That's just a fact of small-scale color mixing. 



* Unlike the 2-pack "have to wear a moon suit" mil-spec stuff you see on new airplanes, which is loaded with PBA or isocyanates, and doesn't work in cans, since you have to mix 3-4 chemicals and then spray it within an hour.  It's pretty good primer, though.