So far as "protecting the paint" let's just do to guesstimation scenarios.
Engine temp= let's just say 220°F or so.
Now how hot will the hood get with the engine cold and just baking in the sun?
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With black paint on a sunny hot day like 95-100 deg. F the black surface of the hood can get from 140 to 160 deg. F. possibly more, basing that on my recollections of temperatures of smooth black asphalt on a hot sunny day. It's not so much the heat from the sun that damages paint, it's the ultraviolet rays that can break down some of the chemical chains or molecules in the paint and cause it to oxidize from weather more rapidly, (water oxidizes paint as does atmosphere). I'm not sure what happens with the clear coat (if it has a clear coat) or the properties of the clear coat and what damage it might sustain from 'baking in the sun' along with other external factors. Eventually as with paint it will oxidize from weather, fade as surface molecules are lost to the weather. I think that IF there is moisture underneath or scratches that will allow moisture to get below it the coating might peel from heat cycles (baking in the sun as you put it in contrast to cooling at night) Those are significant temperature cycles which are a form of stress on the integrity of the paint/clear coat.
As far as engine heat? The exhaust can be pushing about 1,500 deg. F and manifolds can get that hot. In fact, as I recall 1500 deg. F is the lower critical temperature of iron, the point where iron becomes workable (the grains begin to slip against each other) allowing it to begin to be worked by bending, hammering etc. This is also where you want to preheat cast iron before welding it (pre-heat to dull cherry red) then after welding peen it while it cools to relieve shrinkage stresses. If you don't pre-heat and peen the weld it will not hold up- the shrinkage stresses will make the weld area brittle and it will break. Brazing is good for some repairs and easier to do. Iron melts at about 2600 deg. F. (all stuff I remember from 35+ years ago).
Check out this manifold approaching 1560 deg.F on this small turbo diesel on a dyno running biodiesel fuel. (esp. near end of video)
http://www.youtube.com/watch?v=g4X9DJAcVMo
Anyhow, the cast iron exhaust manifolds are thick, conducting a lot of heat from the exhaust and dissipating it so you might not see them turn a dull cherry red on the outside, if/when they ever get that hot on the outside. Suffice it to say they are very hot while doing hard work. 220 deg F. on a temp gauge is the temperature of the coolant, most of which is being conducted from metals around the combustion chambers including the heads while most of the combustion heat is exhausted to be further dissipated until it leaves the exhaust pipe. I'm sure you all know this already, it's 3:15 AM here, I'm very tired and just typing in auto-pilot mode, sometimes I get in a half dream state and wander off topics (bad habit/insomnia)
When exhaust manifolds are very hot and the truck is shut down, what happens then? No more air flow from truck moving, nor from fan etc. to move heat from the engine compartment. The heat radiates out from the manifolds (and engine) heating the air which then rises straight up. I don't know how hot the engine compartment can get but I'd say it can get very uncomfortably warm in there.
Can it affect paint on the hood? I don't know for certain but over a long period of time, cycling from hot to cold it could. I do believe I've actually seen evidence of that on a number of occasions. Paint jobs were not all created equal over the years either. There would be some expansion and contraction of the steel from heat/cold which could, along with other factors mentioned in this topic (or not mentioned like dirt and dust and being blasted by it while driving and tree sap is another good one- pollen might be acidic- I don't know) all contributing to affect the paint eventually- I'm just looking at technical considerations even small ones that could affect the paint.
