I want to replicate a very smooth concrete surface, but my foam is kind of hairy and fuzzy, any suggestion on how to smooth it out?
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Put a coat of wall filler or plaster over it, and sand that smooth when it’s dry.
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What Jakko said. Attempting to sand the foam itself will likely make it worse. You need to seal that surface.
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I want to scribe in some concrete panel lines , so do that 1st ? Then seal?
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Do both on scrap. See which is the better method.
What are you replicating? I had some success making foam paper backed foam board look like concrete just using spraypaint.
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run a flame across it?
If you scribe lines first, you will need to make them deep and wide because the filler will — take a guess — fill them 
You’ll have much more control if you put a coat of filler over the foam first, and make it thick enough that after sanding, you will still have enough thickness for the depth of the lines you want to scribe. This also has the advantage that if you get it wrong (too deep, in the wrong place, veer off, etc.) you can just add some more filler to repair the problem and try again once it’s dried.
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