Do you always need to separate a string of adjectives with commas?

The short answer: No.

Here’s a perfect example of when you don’t have to. Consider the phrase “blue polyester uniform pants.” (Thanks to Doug Metz for that!) Would you say “blue and polyester and uniform pants?” I sure wouldn’t. They’re blue polyester, and they’re uniform pants. Take it further. Would you say “blue polyester and uniform pants” if you were talking about that pair of pants? Again, I don’t think so.

The classic phrase often used as an exemplar is “little old lady.” Would you say “little and old lady?” Doubtful. Even if you add another adjective, you still are unlikely to use commas: little old blue-haired lady.

If you wouldn’t use “and” between the adjectives, you don’t need to use a comma, either. It’s a simple test that nearly always works. (I’m hedging a little because I’m certain if I were to make a definitive pronouncement, someone would comment “But Karen . . .” and blow it all out of the water.)