I mount and cut out the bodyshell, then use a small drop of washing up liquid with cold water, wash out, rinse with cold water, then dry it with kitchen towel (use a branded product the cheaper ones leave white bits behind) once dried then use methylated spirits on a fresh new bit of kitchen towel to get rid anymore contamination. after that i apply liquid mask if you use solvent based paint you will need to apply around 4 heavy coats of liquid mask due to the solvent trying to eat into the liquid mask but if you use water based paint i use only 2 coats of liquid mask
HTH
