Polygon Outlines in QGIS

I had an effect of an outline around the grid in the Glasgow Commercial Property Density heatmap. I really like the effect, but to create it I had to save the grid itself as a new file, dissolve it and then symbolise it. There should be a way to do it simply through symbology. When working on some categorised symbology I though symbol levels would be the way to do it, and QGIS 2.6 actually made it possible.

I am not a huge fan of how QGIS deals with polygon boundaries, shared boundaries usually become thicker than non-shared boundaries. But we can still achieve some really good outputs.

We will start out with a simple categorised symbology.

I would like to remove the borders, but that results in very small gaps. We can tell by turning the project background black.

So I will set a narrow white border, which is very unintrusive.

Now for the QGIS 2.6 touch. We can now turn a graduated symbology into a rule based one, simply set up a graduated symbology and then select rule based and your rules will be automatically generated.

For the border we create a catch-all rule that has our desired border.

Resulting in.

But then setting up our symbol levels.

We get the desired result.

Final output.

Desired result with one dataset, excellent.