The window concrete surrounding wall has been painted with waterproof liquid before painted with Dulux weathershield. It is also done with a ledge to avoid water ponding on the window sill.

An external paint can prevent seepage?   

External paint is a weatherproof material, is not a water proofing material.
Same theory as you do not use a weather proof camera at swimming pool.

Of course swimming pool and external wall is different in nature.
What I am trying to explain is the different of 'weather proof' and 'water proof'. And external paint is fall under weather proof category.

A so-called waterproofing specialist contractor suggested the following with quoted 3k+ with 5 years warranty, to do something like on the pic:

1) Apply waterproof cement on the window frame cement surrounding
2) Apply waterproof liquid on the surrounding which penetrate into the window frame holes
3) Take off existing sealant on the frame and sill joint and reapply back new sealant

Is this reliable?

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