fibreglass basics i’m not sure on

Hey guys/gals etc.,

this is my first post, i've been researching fibreglass for months now and have found pleanty of sites where people have shown their workings for fibreglass etc. but i am confused on several topics.  Firstly after stretching and stapling/gluing etc. the material over the desired mould, panel, whatever, and then soaking the material in resin, does the material then become part of the panel, box etc. or are u suppost to remove it, cut it out, whatever when the glass is down? and secondly... does the fibreglass be cleaver and stick itself to the existing panel, or mdf or whatever u r fibreglassing on to? say for example i modified a door panel, and fibreglassed in a tweeter holder onto existing plastic parts, would the fibreglass bond with the plastic to hold it there... or do u need to attach it some other way.

any help would be madly appreciated as i am planning on attacking my mates car with glass soon. cheers in advance

tomy j