John,
For large gaps, I used expanding construction foam. Wind Loc makes a gun that allows very precise placement of the foam. The good part is that it is easy to remove if ever necessary, and adds no weight. I used it for the gaps around my front wing brackets (on an ES-P).
From: John Barrett [mailto:2thman1@gmail.com]
Sent: Thursday, September 19, 2013 4:44 AM
To: lml@lancaironline.net
Subject: Re: [LML] IVP pressurization
Many of you have supplied very useful tips in response to my questions. Since then I've found some substantial leaks that are probably the issue. I wonder if others have had leaks in the tunnel under the spar box that hydraulic and fuel lines go through? I have a significant one here and I fear I will have to stop it in order to get adequate pressurization. Has anyone else dealt with this after the lines are all in and the a/c is flying? I think I can seal up the tunnel front and back but it will likely take a good bit of some kind of goop to do it.