I had HPC in Oklahoma City do the coatings on the cast iron intake and exhaust manifolds for the 272 in my ’55.  The intake and one exhaust manifold has over 100K miles on them.  The other exhaust manifold has about 55K miles on it due to the original exhaust manifold cracking along the way and having to send off another for coating.  The intake manifold has held up very well at this point with very little signs of deterioration.  The exhaust manifold with over 100K miles on it is now showing signs of flaking but considering the amount of heat soaking this manifold took early on, I’m not surprised.  The exhaust manifold with 55K miles on it still looks very good but this manifold did not see some of the extreme temperatures or heat soaking that the other manifold did.

Before sending the parts off for high performance coating, I did go over them with a grinder and sander and smoothed them out which makes a big difference in appearance after coating.

For my headers, I’ve been using Jet Hot but their coatings are also sensitive to heat soaking and will tend to dull the finish if temperatures are allowed to get to the extremes.  Dyno testing takes its toll on these coatings due to the lack of air movement so when possible, I have the headers coated after testing and not before.

I got some price quotes back - looks like $200 - $220 for a pair (plus shipping). There is an HPC location 80 miles up the road from here at Chandler, AZ and a Jet-Hot at Tempe. 

It also looks like I'd better clean them up real well and dye penetrant check for cracks before spending anything extra on them.

I have personally worked on large scale mining machines that have operated in excess of 140,000 hours - they have been rebuilt many times, sometimes with care and sometimes not. With a little "art work", maintenance and common sense improvement, they perform just as the new ones (only smaller). We are doing the same thing with our toys on a different scale .....

You mentioned having to use a bit of grinding and smoothing prior to coating - is the "tooth" of the cast iron exhaust manifods a basic part of the flaking problem? Or was it the initial high temps you mentioned? Or both? I can understand that "high points" are going to be hard to get a coating to stay on. I'm not really looking for (or expecting) a "chrome" like finish on the exhaust manifolds - more like dull brushed aluminum. If it starts out shiney and dulls down I won't be disappointed. I'm just looking to stave off the rust and maybe get some cooling effect out of the fancy finish at the same time.

Although 'toothing' may be a contributor to the flaking, it's minor compared to the heat soaking.  Grinding and sanding in my case were simply done for appearance purposes.  Early on, this engine did considerable idling in heavy traffic and the underhood temperatures were to the extreme.  The headers on my race cars hold up quite well in a racing environment where air flow is always across the headers but whenever dyno testing, the dead air around the headers promotes higher temperatures that ultimately dulls the finish.

Right now I am awaiting another ECJ-9431 (LH bird manifold) to check out, as the one that was running on the engine showed a crack when tested with penetrant. It is a real crack not a false indication (#$^@*)! It isn't un-repairable - but I'm not too keen on coating a less than perfect specimen. The RH side checks OK. 

Is there a weld procedure published anywhere that is specific to the Ford cast manifolds? In particular - the preheat and suggested filler material. I have a manifold (RH) that has obviously been repaired - and with some finish grinding and close inspection it looks like it will work as a spare. Either "someone" knew the drill and how to do this - or there are some tech spec's I haven't found. When we fix alloy steel castings at our place, we usually take the part up to 450° F soaking, wrap and shield everything but the repair site - then armor up the welder himself with a reflective guard. But we usually know exactly what the casting is made of and other important details. 

      A preheat with a torch and a high nickle rod works on the intakes. I'm guessing (?) that those exhaust manifolds are gray iron too. They didn't start to specialize materials until a number of years later. Now they're all hydroformed weldaments anyhow.