What I learned today in board sanding school:

1) Stay in school and get a good education. You don't want to do this kind of work everyday. This is really hard work, but doable.
2) The right tool matters
3) Find a young strong kid to do this kind of work and see #1

Prior to doing any glass work on my hull I needed the balsa surface to be more fair than it was after grinding the old resin off. So I used some #407 in epoxy to make a soft fairing compound. I think I made it a little stiffer than ideal, but I was afraid that working overhead it might sag. Being too thick made using my cedar fairing boards ineffective because you couldn't pull the filler across the large surface. Anyway it went on ok and this morning I started sanding it out. I bought two different sanding boards just for this and future adventures in sanding. The more inexpensive one is nice on the hands, but it is way too soft to be useful on a large compound radius. I think it may have its place in a tighter radius, so it will sit on shelf for another day when that is needed. The good tool is the one body and fender guys use and you can manage how stiff or flexible it is by changing the number of rods in the base. This one works really well and is easy to hold. It has a thin metal base that holds the self adhesive 36 grit paper really well and the base can be cleaned when necessary.











So the low spots will get another light coat of filler and then the glass work can begin again. It should also be noted that the Divinycel can be sanded directly to reduce high spots. The reason for the slight variations in height on the new core is simply that the inner skin was slightly irregular to begin with and the amount bonding epoxy and the way it conformed to the old inner skin varied slightly from one little block to the next. Main thing is to just get this basic fairing in the ballpark and then once all of the glasswork is actually done you can have another fun round of being taken to sanding school.

Ingersol Rand makes an air power version of that. Harbor freight sells a version of it too. Used it to fair the keel on wildcat. Used it to put a new bottom on a Frers 36 and 2 J 80's. 18" long by about 2" wide. You can Finish sand with long board

I had an L cheapo version of a board sander that died a few years ago. I was thinking that a board sander would want to flat spot the compound curves. Was that a problem or did you use it to get close before going to something with a flexible board?

Not a problem on the keel. We marked the % points (95, 90, 80, 70, etc) at the 5 template heights and drew lines down the face of the keel. We kept the sander in the vertical plane aligned with the % lines.
For bottom work - don't try to be to aggressive. No more than 80 grit, 120 would be better. Use for rough sanding. Then long board to finish

A new board sander is on order and I will track down some finer grit paper. For sure this thing will help with the keel work I have to do and I will just test it on the compound curves of the hull. I also plan to use a thinner fairing mix on my next go round to see if I can use my fairing boards better.

One other bullet I should have added to the first post was that a full face mask respirator is absolutely required for this kind hull work. I started out way outboard and thought my regular dust mask would be fine since I wasn't directly below the hull, big mistake. In just a couple of minutes work my eyes were screaming at me, saying stupid, stupid......

Ok I have been having more fun with these board sanders. The pneumatic flat board Ingersoll Rand does work ok, but you do have to use a higher grit with it and it takes time and that is not a good thing when you are working over your head.. You can't even hold onto it when it has the coarse grit and if you could it would flat spot pretty quickly. The AFS bendable board shown above with some 36 grit really gets it done, but absolutely kills your arms in the process. So I am using both, the flexible board with the coarse grit to get the high spots down quickly and then work with the pneumatic board sander with 180 grit to kind of move around and finish an area. In either case this is serious hard work, but I am able to maintain the compound curves of the hull and compensate for all the irregularities caused by grinding the end grain balsa free of the polyester resin.

I have one more light coat of the fairing compound to apply to all the low areas I am working and then it will be onto the glassing.

After reglassing this hull I fully expect that another round of fairing is going to be required. That is something I do not even want to think about. The only good thing here is knowing that all of this epoxy will forever seal the exterior side of this balsa.

The high temps and extremely low humidity have done an excellent job of drying the balsa too. No problem at all getting most of it below 7% and I will live with a couple of very stubborn areas that are 12%. Areas that will not go willingly down to 12% are getting removed and recored. And there are a very few bits that even with the resin well ground off do not seem to want to dry out and that has bugged me, but the time it takes to cut out a small bit is less than the time it takes to keep dinking with them.