Painting a treated fence. Paint keeps washing off

mpennn

Just bought "Ronseal Fence life plus Cornflower Matt Fence & shed Wood treatment, 9L" - Water based
My fence had already been painted, from what I'm guessing with a waterproof paint... oil based?
I've done one coat, not too bad. Left it for the required time to dry. But... tried doing a second coat, and it's just rubbed off. Read some reviews online, and it says how it just washes off. 
What are my options? 
Can I...
Add a clear oil based preserver into the paint, so that it won't wash off 
or
Reading online - I've seen that adding PVA wood glue will work


Product information
You can apply this opaque shed and fence treatment from the Ronseal Fence life range to all rough sawn and smooth planed sheds and fences, using a brush, to colour and provide protection.
It takes 1 hour to dry and will cover up to 6 m² per litre on average.
Colours & protects rough sawn & smooth planed sheds and fences
Suitable for Wood
Manufacturer's Guarantee: 5 years

Davesnave

The instructions state it can be applied to rough or planed wood, but painted or varnished surfaces aren't mentioned because they are likely to resist the water in the product.

You cannot add an oil-based preserver to water-based paint because oil and water don't mix. PVA is also not fully waterproof, so attempting to stick it to the surface with that as an additive may not work that well either.

It seems you have obtained the wrong product for the surface you are trying to colour.

My experience with this product about 12 years ago was positive and the storage box I painted is still looking good.