I've been trying to paint my fence today with cuprinol sprayable in dark brown.

It's featheredge fence, half of which is new, half is old. The new part unsurprisingly looks great but the old section used to be painted blue, and try as I might it will not turn brown. Not even with multiple coats of it. Some of the blue has weathered off but the rest is still right in there.

Does anyone know, short of replacing it, how to turn a blue fence brown?

Paint it

Its probably cause the existing paint is water proof and therefore resisting the new paint. Perhaps a light sand first?

Nah it's jut paint proof, the brown goes over it and seps through the blue until it's blue again, tried sanding it yesterday before my post, didn't work