[How to fix thin mesh walls?]

Thanks for these suggestions. I have been trying different speeds and slowing it down seems to improve the surface but I'm still getting spiral ridges. I'll try different layer heights.

[How to fix thin mesh walls?]

I'm new to 3D printing so I'm probably making a simple basic mistake. I have a new Monoprice MPminiDelta printer which I thought would be a good introduction to 3D printing. 

The test print g code file that came with the printer prints very good quality copies. I have downloaded several stl files from Thingiverse and run these through Cura to print but the problem I am having is that all of these have a wall that is like a thin mesh, not solid. I've  tried making the wall thicker and a few other things but they all print with a thin mesh wall.

What am I doing wrong?

 

The first image (white cat) is the g code file that came with the printer. it prints with solid walls.

The second and third images (purple dinosaur and white unicorn) print with mesh walls. All of my stl files that I have created with Cura print with these mesh walls.