Not sure if anyone out there knows much about optical brighteners. I don’t, but I think if they’re for epoxy they are different from your normal run of the mill dye? Any idea where you can find them in blue for epoxy? I don’t think RR sells it separately and CE is not available in this country and I’d like to give it a whirl.
I cheated on a blank that had some bumps and marks and prepared a mixture of white water based acrylic paint and microballoons/Q-Cell/Microspheres. Which is essentially what you have as Red Devil spackle, it went on really cleanly and sanded down very smooth.
I was actually using it to test if it could also be used to seal an normal PU blank.
If your board is EPS you’ve likely already sealed with this anyway.
Now I know this isn’t an optical brightener for the resin per se, but the board still looks bright white though I don’t ride it much.
Sorry TOALLY MISSED the word blue in your post.
You might try powdered fabric dye, it’s called Dylon here or you could give acrylic powdered artists pigments a try. I helped glass a mates board and he used yellow acrylic artist pigment…but he steadfastly refused to purchase more pigment and the end result looked exactly like urine…I laughed!
BTW titanium Dioxide is a standard optical brightener, used in lots of paints and inkjet papers among other things, but it is very white, no idea about it’s solubility in resin or where you’d purchase it.