How to get uniform paint cover over filled surfaces / old plaster

[MarcoM]

Hi,

What is the best way of fixing patchy paint on a surface which has new filler and old plaster on it?
The filled area seem to have a different shade compared to the older plastered areas, particulary under direct light.

Is lining paper the only solution to this or just apply as many coats as needed till the paint looks uniform under direct light?

Or cover it with Gardz, bearing in mind the expense particularly in a hall and landing scenario.

Many thanks