| Assuming that this is a plastic kit for the Airfix Boeing 757, I would imagine that what you are aiming to do is mask off the windows ? most airliner kits come with those dresdful plastic inserts which are added prior to assembling the two halves of the fuselage together,why not forget these and use a system that is proven and above all looks fine,assemble the two halves of the fuselage,without installing the clear pieces,after masking up,filling joint lines and painting any demarcation lines then the windows can be tackled,to do this purchase some 'Kristal Klear' it comes in a bottle as a liquid,this remarkable material flows into the voids of the apertures and when dry sets like clear glass looking far more realistic than pieces of plastic stuck from the rear.
I hope that this is what you are after as your description is a little vague ? if this is not what you are after get back with a bit more detail and we will assist you further.
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