Dave,
The In line air sander is from Harbor Freight Sales. It ranges from $19.95 to $39.95 if you are going to do much sanding definitely buy the $20 two-year free replacement service protection. I'm on my 3rd and 4th sander. The first two blew out in about 10 hours. I'm thinking that if you drop them that is certain death to the sander. The third one was going good then I started to sand the edge without sandpaper (stupid me) on the foot and ripped the rubber up. So I bought another one as a spare. With the service plan they are really good about replacement. At a regular body shop supply store you would pay $200-$300 for a sander so I don't mind going through a few and we have a Harbor Freight in town.

I start with the in line air sander to rough out the surface. The bog gets very hard so I can get away with 40-grit sandpaper and the air sander. That may be good for several applications of bog as you start to shape it out. You can remove a lot of material so you have to keep it moving evenly over your work. I use the pencil to scribble all over the surface. Then switch to the long board. The low spots will still have the pencil carbon so you can see where you need to fill or sand more.

At a point you switch to the long board you can tell when the air tools have reached your maximum effective point. You will be at a point where it is not getting any smoother or you are just adding bog and sanding it off. Some people stop at this point, then some carry it out further. I’ve been around this type of work as a hobby for 30 years so I can get close by air tools. But 20 foot long you need to board it out. You can buy the sandpaper in rolls to save costs then just cut your own strips it is sticky backed so you do not have to mess with the clamps.

Mike