Blue tac and paint lifting

I am trying to paint a tri colour NATO scheme on my Bradley and I am using Blue-Tac to map out the areas I don’t want to overspray. However, I find that the Blue-Tac is lifting some of the paint that was applied. (Painted about 72 hours ago).

I was wondering if I should put a barrier over the color (i.e. future), Hoping that this would make the Blue-tac easier to mould to the contours of the camoflouge I want and not left the paint…

Also, since it takes a while to finish this job, I may have to leave the blue-tac on for a few days before I get to mapping out the areas and painting different colors. How long can this stuff be on the model before it adversly affects any under coats. I noticed that the parts I did pull off, it left an oily residue.

Has anyone done that,or what is the easiest and best technique.