RE: Microballoons and other mysteries
Here's some good advice on thinning epoxy [link=http://www.seqair.com/skunkworks/Glues/WestSystem/Thinning/Thinning.html]West Systems[/link]
The light tan you have found could also be fairing compound, on the [link=http://www.westsystem.com/]West[/link] site as 410 Microlight.
For heating I normally heat the wood a little with a heat gun (hairdryer ok), thins the epoxy and really allows the epoxy to penetrate the wood.