I've learned my lesson @Yankee back in the day. In my opinion never allow a majority of shares of corporations you plan to keep on the open market. Someone will buy them eventually, usually a hostile player who is looking to do economic harm. Never sell a majority of shares in a mining corp especially.
If you want 250/250 upgraded public corps, speaking to anyone, I suggest owning 20% of shares in your country. Then splitting the other 80% between two other countries in your empire in their investment funds. This way you still own 100% ownership without the risk of a hostile takeover. Common knowledge on the forum, but wanted to bring it back up to any newer players who may make that mistake many have before.