Step 3: Applying the mask
You will now need to print the solder mask that will cover the board and protect the pads you wish to solder from being hardened in the sun. If you are using Fritzing to design your PCB then when you export it it will create a solder mask PDF you can print as well.
If you use a laser printer to print your mask then make sure you use the highest quality possible so it has the most toner on it to create an effective block for the UV light from the sun.
Cut your mask down to size so it is the same size as your PCB.
Once your PCB is dry it's time position your mask over your board and then use a sheet of glass to push the mask down flush on the board. This will avoid the chance of shadows on the board protected things you did not intend.
Place it all in direct sunlight for about 30 minutes.
Now the UV light should have hardened all but the areas protected by the mask, take it inside and remove the mask and lightly scrub the pads with turps using a toothbrush.
The non exposed areas should wash away. Rinse the board in water to remove any excess turps. It should now be ready to solder.