Rendering chimney stack

[Dee]

One of the stacks built in commons has blown the paint after 15years. I did prep the rest of the house but not the stack hence the mess it's in now.

Looks like the surface of the bricks have truly blown off. I'm not going to the expense of rebuilding it do I want to render it.

I'll be hiring a boom lift for the reach but I need to know what mix to use and basically how to render a stack!

[Russell griffiths]

Better pics needed

but I would not render it

take the top 3 courses off and either rebuild those, or just re cap it at that lower height. Does it need to be a specific height for it to function correctly, what purpose does it serve. 
replace the low down blown brick and give it a bit of a re point. 

[Dee]

Can't get high enough to photograph.

Alas I can't afford a builder to reduce its height and my brickie skills are lacking. What's wrong with rendering,?