A simple way to clean corrosion from a twist drill bit is to let your power drill help you. While applying oil to the bit at its point of entry, drill repeatedly into a block of hardwood secured in a vise. Friction of the bit against the wood and abrasion of the wood chips will provide a scouring action to clean up the bit in a hurry.
When hiring a contractor, insist on a complete written contract. Know exactly what work will be done, the quality of materials that will be used, warranties, timetables, the names of any subcontractors, the total price of the job and the schedule of payments.