The spoiler looks like one of the "80's" rubber ones? If it is then it will have years of polish, back to black type products ingrained into it. These products can play havoc with release agents when taking moulds.(I learnt the hard way) Give it a good steam clean and plenty of panel wipe. Depending on the cure time of your tooling gelcoat you may want to add a little more catalyst and some heat lamps to speed things up. The longer the gelcoat takes to cure the more time it has to "attack" the rubber of the spoiler.