The most recent entry into the covering systems market is the  Stewart Finishing System that uses waterborne technology  to apply polyurethane coatings to the fabric. The glue used  in the system is water-based and nonvolatile. The Stewart  Finishing System is Environmental Protection Agency (EPA)  compliant and STC approved. Both the Stewart and Air Tech  systems operate with any of the approved polyester fabrics  as stated in their covering system STCs.  All the modern fabric covering systems listed in  Figure 3-9 result in a polyester fabric covered aircraft with an indefinite  service life. Individual preferences exist for working with the  different approved processes. A description of basic covering  procedures and techniques common to most of these systems  follows later in this chapter. Ceconite™, Polyfiber™, and Superflight™ are STC-approved  fabrics with processes used to install polyester fabric  coverings. Two companies that do not manufacturer their own  fabric have gained STC approval for covering accessories  and procedures to be used with these approved fabrics. The  STCs specify the fabrics and the proprietary materials that are  required to legally complete the re-covering job.  The aircraft fabric covering process is a three-step process.  First, select an approved fabric. Second, follow the applicable  STC steps to attach the fabric to the airframe and to protect it  from the elements. Third, apply the approved topcoat to give  the aircraft its color scheme and final appearance.  Although Grade-A cotton can be used on all aircraft originally  certificated to be covered with this material, approved  aircraft cotton fabric is no longer available. Additionally,  due to the shortcomings of cotton fabric coverings, most of  these aircraft have been re-covered with polyester fabric. In  the rare instance the technician encounters a cotton fabric  covered aircraft that is still airworthy, inspection and repair  procedures specified in AC 43.13-1, Chapter 2, Fabric  Covering, should be followed. Re-covering an aircraft with fabric is a major repair and  should only be undertaken when necessary. Often a repair  to the present fabric is sufficient to keep the aircraft  airworthy. The original manufacturer’s recommendations  or the covering process STC should be consulted for the  type of repair required for the damage incurred by the fabric  covering. AC 43.13-1 also gives guidelines and acceptable  practices for repairing cotton fabric, specifically when  stitching is concerned.  Often a large area that needs repair is judged in reference to  the overall remaining lifespan of the fabric on the aircraft. For  example, if the fabric has reached the limit of its durability, it  is better to re-cover the entire aircraft than to replace a large  damaged area when the remainder of the aircraft would soon  need to be re-covered. On aircraft with dope-based covering systems, continued  shrinkage of the dope can cause the fabric to become too tight.  Overly tight fabric may require the aircraft to be re-covered  rather than repaired because excess tension on fabric can  cause airframe structural damage. Loose fabric flaps in the  wind during flight, affecting weight distribution and unduly  stressing the airframe. It may also need to be replaced because  of damage to the airframe.  Another reason to re-cover rather than repair occurs when  dope coatings on fabric develop cracks. These cracks could  expose the fabric beneath to the elements that can weaken  it. Close observation and field testing must be used to  determine if the fabrics are airworthy. If not, the aircraft  must be re-covered. If the fabric is airworthy and no other  problems exist, a rejuvenator can be used per manufacturer’s  instructions. This product is usually sprayed on and softens  the coatings with very powerful solvents. Plasticizers in the  rejuvenator become part of the film that fills in the cracks.  After the rejuvenator dries, additional coats of aluminum- pigmented dope must be added and then final topcoats applied  to finish the job. While laborious, rejuvenating a dope finish  over strong fabric can save a great deal of time and money.  Polyurethane-based finishes cannot be rejuvenated. Deterioration of the strength of the present fabric covering is  the most common reason to re-cover an aircraft. The strength  of fabric coverings must be determined at every 100-hour  and annual inspection. Minimum fabric breaking strength is  used to determine if an aircraft requires re-covering.  Fabric strength is a major factor in the airworthiness of  an aircraft. Fabric is considered to be airworthy until it  deteriorates to a breaking strength less than 70 percent of  the strength of the new fabric required for the aircraft. For  example, if an aircraft was certificated with Grade-A cotton  fabric that has a new breaking strength of 80 pounds, it  becomes unairworthy when the fabric strength falls to 56  pounds, which is 70 percent of 80 pounds. If polyester fabric,  which has a higher new breaking strength, is used to re-cover  this same aircraft, it would also need to exceed 56 pounds  breaking strength to remain airworthy. 3-9 Determining Fabric Condition—Repair or  Recover? Fabric Strength