Borax substitute for swirl painting I am currently in the process of swirl painting one of my guitars.

In swirl painting, you fabricate a colored film of oil paint on water with borax. The borax is used to better spread the color film. I think it increases the surface tension. 

Now, since, in Germany, as a private household, you can't get borax any more, I can't get my hands on borax.

Does anybody have an idea of a substitute? My practical chemistry skills have gotten quite rusty after years of theoretical chemistry, so I could use some help here.

Soap wouldn't help because - I think - it would dissolve the oil paint.

Could I use a thinner for the oil paint? Would that give me the same results?

Or would you say borax isn't even needed?
There are lots of alternatives to Borax online for making "slime," but I don't think the rubbery texture will work for you. The closest non-Borax idea I found that might meet your needs is to use either colored Jello or plain powdered gelatin (it's often in the vitamin supplement section of a drug store or pharmacy, or in a grocery store in the baking products aisle), add the amount of boiling water that after mixing well will flow enough for painting, but will dry down enough not to be sticky. 

You should probably experiment with different amounts of water and /or gelatin, as well as different cooling times, before applying, but this is a really simple, inexpensive, legal "recipe" and has the added benefit of being edible.