Alternative to Zinsser Gardz?

[Solyom]

My elderly friends paint is peeling off the wall in large pieces. The wall is covered in a white chalky substance. The area affected is only the top of the stairs where there was a leak from the loft. He had this repainted but obviously incorrectly.
As it is such a small area I was wondering if there was a cheaper alternative to Gradz? Having to buy 5 litres for such a small area makes it extremely expensive.

Thanks for any advice

 

[Bosswhite]

White Oil based undercoat.

 

[Solyom]

Many thanks Bosswhite, will give it a go