David, just to jump in here and answer one of your questions. No, you can't use this straight over enamels. What ever you use to thin oils will attack enamels. I use White Spirits myself, what in the US i believe they call mineral spirits.
You will need to put a protective clear coat over enamels first. But you will be able to do it straight onto Acrylics. But as Karl mentions, its best not to have a flat finish before applying this method. He does it on a semi gloss finish, i prefer a gloss one.