Get all details of the job in writing on the bid and then on the contract you sign with the contractor that you select. Materials that are included in the price should be specified in the contract, as well as the price, and all prep work and labor should be detailed in the contract.
An experienced and qualified painting contractor should be able to give you specific information on Industry Standards for prepping and priming various surfaces. When interviewing painting contractors, ask for the Industry Standards pertaining to your project.