Hi,
We had this discusion a number of times on the German board.
I tried most of the solutions to this problem, including clay (R&G) and polyester Bondo.
The clay is NOT hard enough so you get brushmarks along the centreline of the mold.
The wax also get "damaged" later because of applying the release agent to the plug.
I noticed that polyester (Bondo) can react with the release agent, PVA and W70 (wax),
perhaps because of the heat it produces while curing. [X(]
If you are "lucky", you will only have marks on you plug because of this reaction. If you're not, you
are not able to get the plug out of the parting board anymore... [>:]
What works perfect for me, and I know it takes a little time, is making a mixture of epoxy and talcum powder.
Make it as thick as possible and fill the gapp with this mixture.
Let it cure, take out the plug and sand the epoxy. You will see sanding this is a piece of cake because
the talcum "lubricates". This is better then microbaloons because it leaves not pinholes.
To make this process as easy as possible, I always make an at least two piece parting board.
That's easier to get the plug out...
The result is a rezorsharp edge.
Just try it, you will see it works!
Hope it helps you,
Arne.