Ok, thats all i needed to know.

You need to use it extremely sparingly. Like a mist so fine you can't even see it on the part, it will still stick. Takes a bit of practice because the 3M Super 77 likes to come jetting out of the can. When you get the hang of it you can use it without any blemishes on the surface finish. 

leaves no markes or spots in the gelcoat?

like Fasta said 3M super 77

I couldn't get on with the 3m stuff, now use cytec aerofix and it stays tacky for ages. Almost too long as I use it when cutting off the role to stop fraying and sometimes it sticks to itself when rolled back up

Look for 3M super 77.

another question regarding the spray adhesive.