Craig,

 

I know exactly what you're going through.  When I did my wings, primer seemed like it was not going on thick enough to fill the deeper areas but micro was too difficult to sand out smooth.  I used small amounts of body filler on those in-between areas.  Things are more complicated on your IV-P compared to my ES since your fuselage is going to blow up like a balloon and is much more susceptible to cracking of the bodywork. 

 

When I do micro I mix it really dry, so it hardly holds together.  I wet out the surface and then go over it with a paper towel to remove any excess epoxy.  Too much wet-out will soak into the micro and make it wetter, then it's tougher to sand.  I wet out several inches outside of the area where I'm going to put the micro.  That extra epoxy sort of protects the prepreg from sanding.  The really dry micro won't spread very well, so get it close and grab a heat gun, warm it up and is spreads like butter.  Really dry micro tends to have a lot of pinholes in it.

 

I like to think that micro, even micro with a small amount of flox is the way to go for contouring.  Some of the other off-the-shelf products like Feather Fill work very well too.  You'll go crazy trying to get micro (or other products) perfectly straight.  It doesn't feather like primer does and you can beat up the prepreg if you try to feather it too much. And every batch of micro is a little different, and sands a little different.  I like the idea of just loading on the micro and getting it on the first try.  Micro, sand, micro, sand, etc. is harder.

 

The primer is the best way to get the bodywork straight.  Since primer, micro, and carbon fiber all sand at different rates, I really think you need to really load on the primer, 2 or 3 heavy coats then block sand until you start to get a transparent area in the primer then STOP.  Trying to get the last little low spot with a couple more sanding strokes is were you can introduce wavy areas.

 

I used small amounts of body filler, Piranha putty, where the primer wasn't quite filling, but micro would have been too much.  But only very thin builds of body filler.  It will shrink, but not really a problem if you keep it really thin.  The nice thing about Piranha putty is it sands just like the primer so it's easy to work with over the primer.  I may be wrong about the body filler on a pressurized plane.

 

So don't be afraid to load on the primer.  I probably swept up a few hundred dollars of primer dust from my hangar floor before I was all done!

 

Mike Easley