make rangefinder (HC-SR04) "blind" for some time, with newPing library??

Hello forum. In my project, i need to have a really thin plastic membrane, on top of the HC-SR04's transducers, in order to make them weatherproof..

(they are not going to be submerged, just some chanche that water can get to them cause of rain)

I am using the newPing library.. 40Khz can definetly pass through such a membrane without a big problem... (correct?:P)

the problem is, that even with a very thin plastic membrane, the code returns the distance of the membrane... (0-1cm)... i guess that there are actually 2 reflections... one from the membrane, and one from the actuall object....

is there any way, i can introduce some kind of delay, to make the sensor blind, for echos less than 2cm ??

Should i not use the NewPing libray to do this?? if yes, then can you please give me some advice what code i should write??