Comment by cfmcdonald

Author of the OP here. The "spray" technique was known in the early 80s, if not earlier. It's mentioned in Michael Tomczyk's "Home Computer Wars":

> The solution came in several forms. One way was to embed ferrite balls in the plastic case. Another way was to spray the inside of the case with a metal coating. But the best way was to encase the offending electronics in a small metal box inside the case, which is what was done with the VIC-20. [0]

Why a metal box is the best way, he doesn't say and I don't know. My best guess is that it was more effective/reliable at passing the tests.

[0]: https://archive.org/details/the-home-computer-wars/page/205/...