I recently found a photo of the Ampol No.3 tanker on 24 April 1965 (right in my era) in the Train Hobby Blayney/Cowra Branchline book (page 21) and noted that it had a petrol spill from one dome. Here is an extract from the photo showing the spill. There is also another photo of the same train earlier in the book.
I decided that I would attempt to replicate this and here is the result.
I first gave it a dusting of a pale yellowish brown and then airbrushed some clear gloss in the appropriate area. I didn't use a brush as I felt that it would give too sharp an edge.
Unfortunately my air brush decided to spit and I ended with a couple of tinydrops when doing the dusting, oh well it still looks good.
But it could do with a bit more clear as this closer shot shows.
Here it is with some more petrol stain (clear), the spatter can be seen under the walk way below the first dome.