Get as much gelcoat ground down as you can. A fleck here and there isn't gonna kill you. You need a strong mechanical bond when attaching new glass to cured glass, mechanical bond is about all there is with PE or VE. 40grit and leave ridges to give the glass something to hold onto. If you let the new glass cure, you have to grind it rough again to apply subsequent layers. To save on grinding, do wet on wet application whenever possible.
If you had 1708 coming out the wazoo you could use it for both sides of the plywood and step back to 5/8" or 1/2" plywood. You having fun yet?
AND We want to see some pictures.
