In some areas there are two layers (between the buckles and the adjustment plates). I stick the second layer on there. But that still leaves the backs sticky- it actually helps position the belts and doesn't seem to create any problems.
Now, what I said above is for the tan belts I use to make belts from the tan masking tape. If I need another color, I paint on both sides- after sticking the second layer on. BTW, that second layer comes up away from the first layer beyond those adjustment plates, and leaves a little flap area (underside of that flap should be colored same as belt, of course).
Also, for 1:72 scale, and some 1:48 models, I paint on the buckles and adjustment plates using aluminum or steel plates. 1:32 scale belts require actual buckles, and 1:48 may, depending on how visible cockpit area is.