Henk,

Try reducing the temperature of the emulsion for coating. I really think you should try that before you introduce other variables. I coat paper at 40C, but I coat film at 34C and plates at 37-38C. Adjusting temperature is a serious "trick of the trade."
d

fwiw, I don't pre-heat my coating tools no matter what I'm coating. If you do pre-heat, recognize that is a temperature variable that must be addressed with changes to emulsion temperature.