Before applying the resin, you'll want to make sure the PCB is cleaned and free of any rust. Since mine were CNC milled, I used a scotch brite pad to lightly buff out the burrs and was even able to remove a small amount of oxidization.
Once cleaned, place the PCB down on a piece of blank film. Remove the protective cap from the tube and press your finger against the plunger to dispense the resin onto the PCB. It’s easy to dispense onto the PCB but how much to use will depend on the size of the PCB. Use your best judgement to determine how much resin can cover the entire PCB.
Grab a second piece of blank transparency film and lay it over the glob of resin. This will help spread the resin across the surface of the PCB. Lightly press down on the piece of film and let the resin spread. A piece of glass or acrylic can then be placed on top of the blank film to further evenly distribute the resin.
You can use a squeegee to further spread the resin, especially to the corners and any hard to reach areas. Don't press too hard or you may get thin spots. Avoid causing any air bubbles – slowly move the squeegee across the surface of the PCB. Any excess resin can be scrapped off the edges of the PCB. It's a little tricky to get perfect, so take your time!
Once you're happy with the resin application, place the solder mask film over the PCB and line up the artwork. Alignment is super important here, so you may want to take your time finessing the placement. To give the film a bit more "grip", I added a drop of alcohol to the back of the mask so that it would stick to the blank film better.
Expose UV Resin
Now it's time to expose the resin! To do this, I used a 100LED UV flashlight. You can use a lamp too, just make sure the light source is large enough to cover the whole PCB. To properly expose the resin, you’ll need to find the right balance of exposure time. In my project, I exposed the resin for about 10 seconds with the flashlight about 2-3 inches away from the PCB.
When I first tried this, I started to peel away the film and quickly knew I didn’t expose it evenly enough. The some of resin stuck to the PCB but a big chuck tore off. The size of the PCB, amount of resin and the distance of light source all matters so you’ll need to do a few tests to get it right. After a few trials and tribulations, I found 10 seconds works well with the right amount of resin. Any longer can result in over exposed solder mask.