If you are sure it is an oil based enamel on there and you want to use cellulose, the only recourse will be to first apply a sealing coat between them. Otherwise forget the cellulose and go for an acrylic (water based acrylic not chemical based as are Halfords car paints) paint. Water based acrylics will should cover with no problem.
Thanks for that Brian but I think my question needs some clarification.
Firstly, the varnish on the hull is a modern adaptation of the old Chinese tung oil recipe, ( Le Tonkinois ), which is tung oil with modern driers added, therefore I can only use paints that don't contain things like cellulose, acetones, propane and naptha etc.
Only true Enamels are what I need, of which most so called Enamels are not.
Paints that can be cleaned up using turps or white spirit are usually enamels.
I'm not a lover of water based Acrylics as they tend to be quite hard when dry, a wooden boat needs paint/varnish that is flexible enough not to crack when the wood moves, as it surely will.
So the question remains, does anyone know of a spray can paint that is proper spirit based Enamel ?
Mike.