ok thanks, sounds rather un-obvious
Hi Simon,
Yes, it is not that intuitive the first time around. I use this process to open up the solder mask on bottom tracks quite a lot. When the board is then wave soldered, the tracks have solder deposited over their entire width. This adds significantly to both their current carrying capabilities & to dissipate heat from any attached components.
To accomplish this in DipTrace:
1/ Click on "non signal layer assignment" down arrow (shown highlighted in blue below). In my example below I have chosen the "Bottom Mask";
2/ Click on the "shape button" 3 icons to the left of the above down arrow. Different shape options will then pop up (as shown below). Simply then select the shape you want.
Be aware that when you choose to view only the mask layers (ie you turn off all other layers), DipTrace has a weird anomaly in showing you only your manually added masks. It "forgets" to show you the masks around all your pads & vias etc. The only way to view the mask screen with all the elements shown is to view the final Gerber output.
Altium & Proteus are much better in this respect, in that that both will show you the "truth" without having to look at the Gerber output.