Just to add info it is 6mm plate in the affected area. Thinking I'll cut a square out a bit smaller than the patch and lap weld both sides of it?

 

Welcome to the forum!That looks like steel to me or is it cast iron?

 

I'm betting that rot goes a lot farther than it appears on the outside...might want to tear into it before you buy new steel.

 

Looks like.

 

I'll just mention old stove steel can soak up a lot of carbon from wood burning and heat cycling so it sometimes welds like cast with that thought it may be better to plate over with a round or football shaped plate and weld from the outside I wouldn't use a square patch cause it may crack at the corners after some heat cycling

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What does the inside look like ? Add new plate inside ?

 

The rust areas stretching right out to the sides were just surface rust, still bright steel underneath. I did weld right around the corners in one pass so hopefully that prevents cracks there. I haven't got a plasma cutter yet so a bean shape was going to be too much work haha.

Just need to turn new studs to put under the leg brace and it should be good to go. Struggled with the restarts a bit yesterday, its not pretty but I think it'll do the job.

Thanks for the help