During the last week, awaiting for the parts needed to complete the chassis, I've applied some coats of liquid mask on the inside of the body.
and third coat.
The idea was to coat both the painted and the unpainted parts and after to remove the mask from black parts to complete the black coating before the white. Unfortunately the black paint sprayed by the previous owner was a thick layer that stiffened the liquid mask and it flakes when removed.
Before making trash of my Yota body I decide to apply 3 or 4 more coats of mask only over the already black parts: it worked! Removing the mask became easier and I can go on with the paint work.