Bonding cement on cement

Hi all,

I am doing some patio renovations and have 2 related questions.

1. We have a rendered wall and the bit along the top has blown. It is around 5mm deep and runs between 2 plastic angle beads which form the corners. Before I redo the render, should I coat the cement it will be applied on top of or will it all bond together naturally. Trying to make sure it doesn't separate going forward.

1. I have a concrete base for some steps which needs to be adjusted before I can fit the tiles on to it. I need to build the front edge out around 70mm and the flat surface by around 10 to 20mm. Again do I need to coat the existing or do anything special or can I just pour the new concrete into position using timber forms?

Many thanks

Lee