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DT, a painting and dec job requires a precise plan before commencing.
The person carrying out the job will want to make sure the customer is aware what needs doing, what materials are required and - more or less - how much material (cost-wise), and approximately how much time is needed.
If you have to repair cracks, holes, or do other preparatory work prior to painting, such as applying special coats (for example against damp stains), you''ll want to make sure the customer is aware.
And one paint has to dry before the next coat or type is is applied.
And usually, one coat is not enough, unless it's the same colour as the old one. And so on and so forth.
If in doubt, just ask a painter - without making known your intentions - to paint one door, or one wall somewhere.
If it comes out right, with a clean finish, and the price is right, you can casually mention that some more needs painting, perhaps a room, or all the doors, or whatever..