after demolding a part. i desided to go into the 2 part mold i have.
Well, i KNOW for sure all my edges were sealed with sealant tape + bag. BUT still… loosing air somewhere. and i believe it was the part b/t the 2 molds. now, how the heck will i ever make a part out of 2 parts?? =(. i put tape on it before doing anything.
Most of us that use a two part mold with either make the parts in each mold section and then join the parts or you use clay in the flange area to help seal it.
Ive heard of scotch invisible tape being put along the seam line as well.
On bigger molds where you can shoot your gelcoat into it no problem (multi part line mold for big parts) its because the mold is too heavy and hard to get the part out of it without the part line removal.
In those cases the gelcoat will seal up the part line.
you are correct. when I spray my gel it seals up the parting line. But most of my multi piece molds I just make each section at a time and use both sections to help me put the part together
weird then, maybe it wasnt the parting line. i used a good amount of gelcoat, and tape on the line. hmm. maybe i had a bag leak indeed! GRRRRRRRRRRRRRRRRRRRRRRRR
Envelope bag.
yeah thats my next move. =( i dont want to waste bagging thou. i guess its better then carbon + leaks! 
Apply sealant to the flanges before joining them. Seem like common sense to me, but I suggest it anyways.
Clay works well to smooth the parting line after the mold halves are put together. Wouldn’t count on it to provide an airtight seal.
Mastic gun dand ecorators filler is a handy standby for any leaks be it flanges or bags.
im just going to do a whole bag. the part came out for the most part great. minus the air that went inside, it created pockets of uncured resin b/c of the air. oh well… it was all for testing anyway.