<p>I'm using Cura to slice my prints. I've noticed that when printing the bottom layer (and also the top layer, if it's flat), it first prints three walls, then fills in the middle by moving back and forth in straight lines.</p>

<p>I've noticed that for my parts, the walls look much nicer than the zig-zag pattern in the middle, and it also seems that the zig-zag part detaches from the bed quite easily, whereas the walls don't.</p>

<p>My parts would look much better, and possibly also stick better to the bed, if I set the number of walls to 100 or so, so that the parts would be entirely filled in with walls. But then the parts would be completely solid, which isn't what I want. So what I want to achieve is that the bottom layer (and if possible also the top layer) are printed as if the part was composed entirely of walls, but the other layers are printed with three walls as normal. Is this possible in Cura?</p>

I found the answer myself just after posting - I'm posting it because it might be helpful to other Cura novices.
There is a setting for this, it's just that it's not shown by default. In print settings, you have to click on the three lines next to the search box, and select "Show All Settings". Then you can find a setting called "Top/Bottom Pattern". Setting this to "concentric" does what I described.
Actually this setting affects not just the top and bottom layers, but all layers that are part of the top and bottom shell. This seems like a good thing, but if you really want to affect
just
the bottom layer, there's a setting "Bottom Pattern Initial Layer" that does this. There is also a setting under "Experimental" called "Top Surface Skin Pattern" that I think does the same for just the top layer.
In addition to "Concentric" there is also a "Zig Zag" option that's quite similar to the default "Lines" mode.
You can also change the visibility of settings in the preferences menu, to make these settings show up by default.