A few years ago I picked up some tubes of squeezable decorators caulk to try out on creating some shapes and lines on some paintings. They turned out quite well and it is really easy to use.
It is quite a flexible substance so it works well on canvas and it is easy to apply. Once it is dry you can paint it (I think I pretty much used only acrylic paint at that time but I expect if you applied a coat of acrylic and then used oil on top it would work perfectly well too) and with the raised surface it is easy to apply something like gold to the caulked bits.
Here are an example of a painting that I sold using decorator’s caulk:
You can use the caulk to make lines or to create outlines of places where you maybe want to put in paint of a different colour. In fact you could create an enclosed area where you could pour in paint and allow it to mix itself.
As I have been recently renovating my house I now have a few tubes of caulk left over and figure I might as well make use of them!