John,
My experience with these products is that they all work well.
Finding out how they will work with other products is the question I have now.
For undercoating and sound proofing I have settled on 
http://www.noxudolusa.com/default.htm
The underbody rust proofing is excellent, the sound proofing is far superior to DynaMat and copies, and best of all the products are sprayable and cleaned up with water.
They are not the most cost effective and you will need a Graco sprayer to buy in larger quantities.
Most of the products come in smaller rattle cans as well.

Waxol and Krown are excellent for inside the pillars and interior seams.
You can get both here in the US, but they are hard to find.
Range Rover groups have steered me to the vendors of them.

Pre-undercoating? If you plan on layering these products....I would do some testing.
One of the ways the rust proofers work is that they don't dry all the way and wick into cavities.
If you layer over such product with another needing a dry surface, it will not turn out as you have hoped.
Been there done that.
Removal of these products are a PITA.

Here are the layers i have used and tested for 5+ years now:

bare metal
POR15
POR filler products
POR (putty) seam seal
POR etching primer 
High build primer
Noxudol Body for lower van and fender wells
Paint
Clear
Noxudol Cavity Wax
Noxudol 1100 underbody

No issues to report.
I feel that I have a very good understanding of how to use and have POR15 work properly.

Please let me know how you make out.

dylan

�

________________________________
From: John C... <trvlr2001@COMCAST.NET>
To: vanagon@GERRY.VANAGON.COM
Sent: Wednesday, January 14, 2009 11:47:22 AM
Subject: E-Tec 521 Waxol Rust Preventive;

http://www.epi.com/pages/waxes-and-acrylic-resin

Hi All,
Is anyone familiar with this Wax Emulsion Rust Preventive Product?
Particularly� the E-Tec 521v?
Is this like or the same as Waxol?
Do you think this would be a good pre-undercoating product?
best,
John C...
SLC UT...