Clean the shell well before you apply the liquid mask, but i dont re-clean the peeled off areas and have not had issues. I would be too worried about the mask lifting.
When cutting the liquid mask use a brand new scalpel blade and cut very lightly into the mask. If you push to hard you will score the shell and it will be prone to splitting if you have an accident.
Some of these pics below show liquid mask on a shell, my cutting it and the resulting transformers text and logo. These came out really well even if the overall body shelll was not really up to my original plan (but my issues with pink paint on this shell are another story).
http://www.oswestryrcc.org.uk/Images...formers/t1.jpg
http://www.oswestryrcc.org.uk/Images...formers/t2.jpg
http://www.oswestryrcc.org.uk/Images...formers/t3.jpg
http://www.oswestryrcc.org.uk/Images...formers/t4.jpg
http://www.oswestryrcc.org.uk/Images...formers/t5.jpg
http://www.oswestryrcc.org.uk/Images...formers/t6.jpg
Personally i love liquid mask. One you get used to the smell when it's drying. I cant comment about the Bob Div stuff, but the stuff i get from Apex Models stinks to high heaven when you heat it with a hair dryer.
Last points: dont get an dribbles in your paint mask, brush them out before it dries. Dont skimp on the layers or you will have trouble getting it off the shell. Find some tweezers from your first aid cupboard and givem them permanent asylum in your painting box.. they are useful you pulling the mask off.
