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Airliners from 50 years ago couldn't make themselves invisible to radar. Still can't.

Turn off transponder. In the US, outside the ADIZ, we don't have a blanket of active radar.

That's fine for secondary, but you'll still show up on primary.

Civilian primary radar in most places isn't that strong. It may or may not get a good skin paint depending on distance, size, and atmospheric conditions.

Primary radar is good enough to pickup a 2500# single engine airplane from 50+ miles away.

I flew from Detroit to Boston a few years back in a 4-seat, single engine airplane after an electrical failure. On a hand-held radio and with no electrics, ATC had a good primary target on me the entire way (at 5-9K feet). On controller handoffs, I would have to turn north for 1 mile for them to confirm the primary target they were looking at was me.

Up and down the Eastern seaboard, I pretty regularly get traffic called out as "primary target only" which also means ATC has no idea of the altitude. Those tend to be low, slow moving aircraft (often without electrical systems).

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