Once you have collected the paint and thinner, meet back up with Gus and move onto the next area. Here Gus will instruct you in the basic uses of paint and thinner. If you have missed this mini-tutorial i suggest starting again so you can see it again.
After making your way past the first two obstacles, you'll come to a bunch of boulders blocking your way. Dissolve the floor beneath them so they fall, then paint the floor back in to cross into the next area.

Thx, if this solution helps please rate :)