Well Cast iron is a totally different animal. My specific concerns about alloy castings is the alloy has a sealed surface,, Like a heat treated camshaft. Surface is hard and inside is soft.
But Cast iron,, That does not have those issues. In my view it totally depends on what the casting looks like and what prev finishes were on it. Is it rusty and covered with scale?? Id sandblast it. Is it covered in a thick coating of paint, or worse,, multiple coatings of paint? Id strip it in a chembath and then soda or sandblast it...
Some people powder coat a cast iron cyl and while I totally get the show boat trailer queen over restored look,,,I dont think thats very smart.
In aviation school, in order to pass each step we had "Practical" tests which involved demonstrating in front of an evaluator a task such as replacing guides, valve job, and painting a cylinder. Back then we used Walnut shell on everything (Now soda media is the std) and then had to use a special zinc chromate primer and then a Catalyzed enamel or Poly black paint.
The trick was to prevent corrosion and totally paint ALL the orifices and between the fins, but not slather it on too thick. This was rather challenging.
While many people use a variety of rattle can paints to some degree of success,, What I use when painting (Since its a related topic) Is I use either Diamont (RM) DE15 etch primer and its a thin primer with corrosion control and etch (Bonding) properties. Lately I use Valspar VP50 as well and mixed at a etch only mix (You can use it as a etch surface primer or thicker as a sanding filler). But in the paint trade its called a "*** coat" which means its only a very thin coating and almost see thru.. You are only applying enough material to provide a surface substrate for the following paint to stick and bond too.
I then follow with a single stage Poly catalyzed paint. (usually Valspar, but I have a couple other brands i like too). Again,, uniform coating, but not too thick.
Alternatively (I know you didnt ask this, but its useful info) I have been increasingly using special coatings. I have a few shops locally that can apply specialized coatings such as heat reflective, or lubricating, or oil shedding. For example piston tops are coated with a ceramic coating, the sides with a moly lubricant, and the underside with a oil shedder.
But I have had a number of castings (engine cases) as well as cyls and heads done in a semi gloss black heat dispersant. It works REALLY well,, reduces heat by a sizable amount by verified testing. All of this has been extensively tested, proven and the results are there. I have several people I know who use this exclusively in racing and have been for 20 years.
This is one shop near me, and great people. There products are tested on the Land speed record racing circuit (Bonneville, El Mirage and Loring, etc etc) Some guys wrote a book years back on
"How to hotrod your motorhome" and again, trannys, cooling systems, brakes, and engines all showed great performance increases.
See :
http://finishlinecoatings.com/While not universally popular,, I know Kenny Dreer very well, Vintage rebuilders and Norton America motorsports. Kenny was using Finishline for coatings on many of his builds for the last 25 years. (He is now retired) and ALL the VR880 bikes had these coatings.