When the arching is distorted, it usually means that one side has been pushed up and the other side has gone down, so the scraping of the casts should be happening equally on both a negative and a positive cast to get the arching back to its original shape by pressing it into the 2nd negative cast. At times the whole process may even need 5 casts altogether.
There is many ways of sealing the cast with different mediums. This is how I was taught at Beare’s. “Mould Makers Soft Soap” or “Antique Wax” are other options.