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.
Comments
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":
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/...