Hi, I'm trying to do a cyanotype on some raw cotton canvas that I bought, using the classic formula found here. However, even after exposing for an hour in sunlight, I got quite a faint print that was light blue in colour. What should I do to improve the print quality? I tried the same formula on normal cartridge paper and it turned out fine, so it could be something to do with the canvas. The canvas doesn't absorb the chemical very well and I have to do several strokes with my brush to get it fully coated - could that be a problem?