[Tigers] Aluminum Heads--a downside?

[Alvin Johnson]

Hi Doc,
Even given "paint-negation," it's a safe bet that painted heads will yield
more HP than any Ford small block iron stocker ever will, no matter how
ported, relieved or polished..
First I've heard of it---how can external paint affect internal operation?
Would the same hold true for an aluminum block?    Painted aluminum heads have
been a favorite street racer trick for many years.
Al J