Regarding David's PCB fabrication process (for #DNA-LAMP Diagnostic Device) I asked if he tried using the black spray paint also as solder mask. While he doubted the paint would survive the soldering temperatures I thought it would. Of course this depends on the type of spray paint (and probably other variables), but I was just curious to see if the cheapest one from my local hardware store would be up to the task. Therefore, in order to find out if spray paint is a viable candidate for cheap and easy DIY solder mask I did a quick and dirty test to see how the paint behaves when heated to typical soldering temperatures and this is the result:
I applied some solder and kept the soldering iron on there for way longer than would be necessary to solder a component. While heating the copper small bubbles formed at the border of the copper area, like the ones indicated by the arrow, but almost all of them disappeared again. I think the bubbles might be due to some residual moisture since I didn't let the paint dry for the recommended amount of time.
The masking wasn't that great, which is why some paint wicked itself under the masking tape between some copper strips as can be seen in the bottom left. These residues seemed to melt when heating the copper but the main layer of paint seems unaffected.
A quick continuity check showed no short-circuits between neighboring copper strips and measuring the resistance proved that the paint forms an electrically insulating layer (no surprise there but I thought I'd mentioned it for completeness). However, this single coat of paint doesn't stand up against mechanical damage very well and can be scraped off quite easily. Multiple coats of paint and adequate drying might be required to address this.
From this quick test I think spray paint might work as makeshift DIY solder mask and I am very excited to see how David's test turns out.