all depends on how they are done Mark, the cheap way is just to apply several layers of primer on them then spray the colour, this will leave the texture slightly like you are reporting.
The professional way of doing the arches is to prime, rub down, prime, rub down, prime rub down, then paint. Obviously more labour intensive and more costly if say a dealer was to do it, but does give a better look/finish.
This could be done by someone themselves if they wanted to reduce the cost of the painting, just buy a second set of arches and do them before submitting the car for the body colour coding, they are not that expensive
Always worth asking a paint shop how they intend to carry out the work so you know how they will look
