While I'm not an expert in Cyanotype, I have done a number of them. In addition to the light source, the paper could have some impact on how much "washes down the drain". I found that a thicker harder paper (like Bergger COT 320) doesn't let the cyanotype coating soak in as much and the emulsion seems to sit on the surface of the paper more than a softer paper. Bergger COT 320 works fine, but I found I had to increase my exposures signfiicantly.