Your decal will settle down and adhere the best on a glossy finish. You need not clear coat the entire car, though, just the area where the decal goes. Future floor finish brushed on these spots works great. Many of the factory finishes will still produce some silvering under the decal film, it is always recommended to gloss coat before decaling for best results.
As already stated it is better to apply the decal first, then weather. If you are doimg a seriously weathered job , you could do some weathering first, glosscoat, apply the decal and then add the final weathering.
On the other hand, if you weqther the car first, and then add the graffiti, it will look like the artist tagged a dirty car’ If you prefer the “weathered graffiti” method, rember to add the car number and reporting marks over the graffiti.