OK, so I dug up my undercoating gun tonight, and tried it out with some gelcoat. It lays it on THICK, and textured, which is OK by me, it's hard to judge when you've over-applied though. I used about 1500ml of gelcoat tonight to cover one end, and the whole window frame, and the whole top frame. I'm guessing it'll take about a liter to do the back, and another 1500ml to do the bottom. I've got a bit of a problem because I didn't add air dry wax to the stuff I sprayed tonight - I figured it would get recoated, but it's so thick it won't need any recoating. I'll have to spray it down with PVA when I'm done, let it cure, and then THOROUGHLY wash everything with soapy water, which will be a serious pain. Oh well. Better that I guess than having to sand and recoat!