The NYFS has to be applied to raw wood. It does not work on painted surfaces. It has to be able to soak into what ever it is being sprayed on. NYFS gives your surface a Class A flame spread rating (according to NFPA 101 you can have a Class B rating if you have sprinklers). Once you paint over the treated wood you lose the Class A flame spread surface rating so you need to use a paint additive in the paint as well. Some people just use the paint additive. It really depends on what your fire marshall will allow.
Sleepy Hollow Productions, LLC / Folklore Haunted House