I ran a production powder coating facility for almost 20 years. There are a couple misconceptions being passed around in this thread.

First off, powder coating is not plastic. It does not 'seal' the surface it's applied to. Powder is paint, with a mechanical bond to the surface it's sprayed on. The bond is not appreciably different than liquid paints. It is not a molecular bond. A cured powder coating surface still has some permeability.

Powder is generally a thicker coating than liquid pint. Normal mfr's recommended film thickness for a smooth finish tends to be in the 2.5 - 3.5 mil range. Some coaters will put it on heavier, but that does nothing to improve the performance of the powder.

Anodizing is a very thin, but very hard coating. It tends to work better on extruded aluminum than it does on castings, mostly because of the inherent porosity in an aluminum casting.

We did a lot of intake manifolds, valve covers, timing covers, engine blocks, motorcycle engine side covers, rocker covers, cylinders, etc, etc. Generally engine heat will not discolor a properly cured powder coat finish.

However, powders a made in a number of different chemistries, for varying reasons. Some are more resistant to chemicals, some more resistant to UV, and other variables. Epoxy powders tend to provide the best chemical resistance, but there are not a lot of fancy colors formulated in epoxy.

The one category powder has not completely caught up to is headers and exhaust manifolds. High temp powders generally can't quite keep up with header temps on a hard running engine. There's also a lot of silicone in the formula for heat resistance, which makes it finicky and more challenging to handle/spray. We'd done some headers that held up for years perfectly while others failed miserably. Ceramic coatings tend to be your best bet for this stuff.

It would be best to discuss with your local coater to make sure they understand the environment the part will see. As for cost, much of the amount charged is for the masking of the surfaces that are not to be coated. Masking involves a more specialized tape than the 3M masking tape you see at body shops. It's more difficult to work with and tough to get to conform to irregular profiles or surfaces. Second cost driver is the actual powder you select. Powders can run from $3.00/pd to $25.00/pd depending on what it is that's used. Material costs comprise about 15-16% of the bill, so you can see that quadrupling the material cost will have an appreciable impact on the final bill.

Finally, talk to your coater to make sure you're clear on how to properly clean/maintain the finish. I actually had a Harley owner come in mad as heck because he sprayed EZ Off oven cleaner on his hot engine and it streaked the gray wrinkle side covers we'd painted for him.