When you are laying you have to sand the last layer to continue with the next. I think if you have enougth wall thickness there is no problem because the next epoxi and CF will stick with it.
About the acetone i don't know. I know you can't use polyester resin because it will eat the polyestirene. Better way is to cut and sand the polyestirene. There is a saw made with nicrom that cuts very well this material.