LR,

Had a similar problem, to a lesser degree. Instead of sanding the rib areas down, I built the sagged areas up with spot putty. Not too much, just enough to level it out, weight gain was negligible.

Fixing it after sanding through can be an exercise like chasing your tail. Most materials you use to seal it back up will only raise the level back up to what it was when you sanded through it in the first place, putting you back to square one.

Your best bet is to hit it with the WB poly for a few coats (I'm glassing my whole plane with it), then sand it LIGHTLY as not to cut back through again. Something else that may help after that is Plastikote "Spot Filler & Primer" to smooth it out a little more. It's a "high-build" lacquer primer that will go on pretty thick, but lightweight and VERY easy to sand.

It does a great job of smoothing out the feathered areas and filling any scratches/weave left over. I use the stuff like it's goin' out of style. You can also use it to help build up the sagged areas a little more.

Neo