Powermill can reverse them but it looks shoddy when the model is opened up. I can't give models like that to clients.

I worked on Rhino for years and that had the function to flip surfaces, you could also set the backfaces to have a specific colour so it would be immediately recognizable. I was hoping MoI could do that too.

Also, sometimes I don't need to make a joined solid, I only pass on surfaces. There's no way of telling what's what...