You can (and I do) use the Nuln Oil straight out of the bottle.
To solve the pooling problem you must first give the surface a very dry coat of matte clear spray. This creates a slightly rough “tooth” to the surface that the Shaders will hunker down into and resist pooling.
Any smooth surface: bare plastic, gloss paint, oil paint, etc. will cause the water based product to pool.
Thanks for that tip. It’s the little things that can make a big difference.
When laying down this matte clear hold the gun or spray can well away from the model and let the spray fall almost dry onto the surface.
This is how to create that rough “tooth” surface I spoke of.
Putting the matte spray on wet and heavy will just cause it it to flow out and create yet again another smooth surface.
Also you may find that spraying it on dry will end up giving you the flatest matte surface finish you have ever seen!
As to creating that super flat, matte finish; I again refer you to the photo above of my Pershing engine deck as sn example.
That OD color is actually the bare Tamiya model plastic just sprayed with a very dry coat of the Tamiya Matte Clear (TS-80 rattle can)
I have never done that before but the Tamiya plastic was just the perfect shade I was looking for and the build went together so cleanly that all it really needed was that final matte clear coating!
Again as to pooling; you might try adding just the tiniest drop of dishwashing liquid to the shader. This will further break down the molecular surface tension that causes the water based stain to pool.