Ask HN: How to find work for cheap to learn a new stack?

I'm curious if anyone has done this before. You're already experienced in an existing stack, but want to gain experience in a new stack. So you find a company to hire you part-time for below market rates. The value prop being that you know how to write good code, but with the knowledge that it will take you a bit longer, hence the cheaper rate.

I've attempted to do this, but thus far have had little luck when reaching out to companies. Has anyone done this successfully? If so, how were you able to convince companies to get on board? Did everyone end up pleased with the setup?