When I've used a hair dryer, I found the image washed off -- I think because there was a layer of moist sensitizer under the dry surface, so the color that formed during exposure wasn't bound in the paper surface or sizing.

Some papers just don't like cyanotype, also -- I don't know if it's buffering agents or something in the sizing, but something converts ferric to ferrous and fogs the sensitizer as soon as it dries. Same thing occurs with some papers on processes that use silver nitrate, such as VDB and salt prints.