If its our green goo and putty then sand back like Matthieu said.  It can help to add another thin layer of epoxy gel or even epoxy laminating resin as a coupling coat. 

If it hasnt gone fully hard, just lost its tack, then you can normally get away with just another thin layer of epoxy as above.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Hmm. The epoxy gelcoats that I use HAVE to be reinforced during the tacky stage, if you let it dry nothing will bond to it and it needs to be scraped off the mould. I Iran get away with it with my polyester gelcoats, but not the epoxies. But I use Airtech stuff, the EC stuff may be more forgiving? 

Thank you for the replies guys.

Is this epoxy or polyester? 

you can brush on some acetone with a cloth or so to make it a bit tacky again... otherwise sand it with a heavy 80grid sandpaper to create a good bond again.

Hi,