In KiCad you need to select the Top Solder layer (actually the Solder Resist Mask layer) add a Filled Zone and make this zone big enough to cover your PCB.
For example:
Thanks very much. I’m using Kicad 8 and there isn’t a Top Solder Layer. There’s the layers in this pic:
Do as above but with the Mask layers.
Me too . . .
or F.Mask
Also answered here: How does solder mask layer work?, and take heed of the warnings about manufacturers and communication! Even in clear cases they may ask you again if you don’t communicate your intent explicitly and clearly the first time.
But the mask opening (maskless board) alone doesn’t mean you get “solder tinning” on the copper. You have to ask for certain kind of plating. Again, it depends on the manufacturer. Gold ENIG is cooler than tin IMO.
When you look at the 3D view it should show tinned copper tracks & pads . . .
I’d just like to point out that unless you have a very good, well-considered reason for leaving off the solder mask, it’s probably a mistake to do it.
Another option (for low copper resistance) that I like is to use bare copper wire and two through hole pads to mount copper wire “staples” point to point on the board.
If you play around with the Board-Stackup (colors and Mask-Color)…etc and the 3D-Viewer pref’s, And place Filled-Cu with Mask(with Filled-Polygon) on top of it (to expose the Cu), you can get this (screenshot). Always best to ask your vendor what they want (in the PCB) for this and to make Notes on a Drawing.
It’s probably an appearance thingy, #baresolderpunk or something like that. 
Personally I don’t care about looks, but if only it were as easy as modifying a layer to make my circuits work, that would be great. 
Perhaps poking your tongue out the other side of your mouth like this: 
while creating your board may help to make your circuits work better. 
I think you are feeling board.
Hi. The reason I want to do it like that is it’s for kids in school to solder up and if it’s too fiddly and difficult it is harder to teach.
Thanks for all the replies
" if it’s too fiddly and difficult"
But that is precisely why you do use soldermask; to make soldering less fiddly and difficult.
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Hi RobK,
Thanks. If you look at the two boards in the first post I can assure you 11 and 12 year olds find the triangular one much easier to solder up. I’m sure there’s probably a happier medium but the board on the left is much harder to solder with the irons we have in school.
Bare Cu is not the easiest to solder to, that’s why folks ‘Tin’ the Cu.
You can buy Tinning solution or make your own. Here’s my Video on making it…
photo of results
But that is because the board on the left is completely covered with soldermask. It should only cover everything except the pads you intend to solder to.
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A novice trying to solder a bare-copper board is going to end up with a lot of bridged pads.
Use soldermask - that’s what it’s for.
The bigest problems with your green board in the opening post are the size of the pads and the solder mask partially covering the pads.
Why the solder mask covers the pad, I can’t answer, but the pads can easily be changed to something more suitable for your young newbees.
Below is a board layout demonstrating examples of various pads that are easy to create.
The Rs are 20 mm between pins, C2 & 3 1.5mm and C1 & C4 2.5mm.
Note the pads with red arrows. These have offset holes. Pad 1 C2 can be of use to increase the copper area if the wires from the cap. can’t be spread.
Note also the purple ring surrounding the pads. This is the solder mask expansion. This moves the solder mask away from the pad. This is best viewed in the 3D image below. Solder mask expansion is set at 15 mil which still leaves some mask between pads on C1 & C4, but removes mask between pads on C2 & C3.
Pad editing can be done in the Footprint Editor or directly on the PCB with Pad Properties.
Solder mask expansion is set in PCB Editor > File > Board Setup > Board Stackup > Solder mask / Paste.
I hope this belated comment is of help.
