Smooth flat finish will often depend on whether the existing wall is flat/even. As that will be your guide.
But rule of thumb would be, prepare area to be repaired by removing loose debris and dust. Then depending on depth of chase you have options:
Base coat and blend in finish coat.
Base coat and joint compound, then sand.
One coat plaster.
The knack is to get the edges of the finally coat to blend in to existing wall.
Water and elbow grease on final trowel.
You may need to dampen exposed wall or even PVA prep before you start!
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local