How to get the paint roller to fix the walls?

This item you need quite early in the game and you'll need keep merging screwdrivers and the items that they merge into until you get the paint roller.

It's a lot of merging, the paint roller is actually the seventh item in the basic tools line of items:

Screwdriver > Axe > Knife > Trowel > Paint Brush > Paint Roller

Just keep merging until you get the paint roller.

You get these basic tools from the Average Box or higher.