You will impress your client if you can speak their language – speak about expectations for the target wall before the mural is installed. To speak intelligently about this concept, refer to the guidelines that I have referenced below.
The below list is a creation from several associations, including: the Association of Wall and Ceiling Industries International (AWCI), the Ceilings and Interior Systems Construction Association (CISCA), the Gypsum Association (GA), and the Painting and Decorating Contractors of America (PDCA).
(Levels 1-2 omitted due to irrelevance in this discussion)
In areas to be decorated with a medium or heavy texture or where heavy-grade wall coverings will become the final decoration, a Level 3 finish is recommended. Level 3 now states, “All joints and interior angles have tape embedded in joint compound and one additional coat of joint compound applied over all joints and interior angles. Fastener heads and accessories shall be covered with two separate coats of joint compound. All joint compound shall be smooth and free from tool marks and ridges.” Before final decoration it is recommended that the prepared surface be coated with a drywall primer prior to the application of final finishes. Level 3 is not recommended where smooth painted surfaces or light- to medium-weight wall coverings become the final decoration.
Recommendation: I would only recommend a Level 3 Finish if you have a low light situation, you are using thick Type II vinyl wallcovering with a heavy texture, and the image /artwork you are using is very busy, or, is free from solid colors.
If the final decoration is to be a flat paint, light texture or lightweight wall covering, a Level 4 finish is recommended. Level 4 has been modified to read, “All joints and interior angles have tape embedded in joint compound and two separate coats of joint compound applied over all flat joints and one separate coat of joint compound applied over interior angles. Fastener heads and accessories shall be covered with three separate coats of joint compound. All joint compound shall be smooth and free from tool marks and ridges.” Before final decoration it is recommended that the prepared surface be coated with a drywall primer prior to the application of final finishes. Gloss, semi-gloss and enamel paints are not recommended over a Level 4 finish.
I would recommend a Level 4 Finish for Type I or Type II, or green wallcovering, with little regard for what the image/artwork is.
Level 5 finish is recommended for areas where severe lighting conditions exist and areas that are to receive gloss, semi-gloss, enamel or non-textured flat paints. Level 5 requires all the operations in Level 4. Additionally, a thin skim coat of joint compound, or material manufactured especially for this purpose, is applied to the entire surface. (This definition is referenced to Terminology, Section II, Page 2 of GA-214 to make the description of “skim coat” clear to all.) The surface is smooth and free from tool marks and ridges. Before final decoration it’s recommended that the prepared surface be coated with a primer prior to the application of final finishes. The Level 5 finish is required to achieve the highest degree of quality by providing a uniform surface and minimizing the possibility of joint photographing and/or fasteners “burning through” the final decoration.
I would recommend a Level 5 Finish for all types of media. This is a requirement for thin media such as Pressure Sensitive Adhesive Vinyl.
With all this said, if you always recommend a Level 5 finish – you will get the best from the drywall installers and painters involved with the target wall creation.