I would like to mask out a design on a mold prior to spraying gelcoat.

It won't affect the release properties at all, but you will want to be aware the masking tape will not stick to a surface prepared with easy-lease. It will hold lightly, but it will come off very very easily. The air pressure from spraying would possibly be enough to blow it off, depending on your surface porosity. What will likely happen is the edges of your tape will lift from the surface allowing the coating to bleed underneath and ruin your lines. 

I have had very poor luck with using any of my masking-tapes on an "Easy-Leased" surface -- But:  You can use a coat of wax to reduce the "action" of the easy-lease" That MAY work ..

I had to place some stripes and cut pieces of "homemade Pro-finish" in a mould, with easy-lease on its own that did NOT work at all the tape dis not stick at all .. everything moved around when setting up for infusion
second time around I waxed the areas where I wanted the tape to stick .  For my purpose it worked .. but to trust it for spraying gelcoat ... hmmmm ....

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

Yeh it's not easy at all. It can be done though, it's something I do fairly regularly. Took me 4 or 5 failed attempts to figure it out though. 

Thanks Composite Masters!!