I have never heard or used the Sika product. I thought they were more a construction adhesive manufacturer, not marine, but I guess they do have marine epoxy. Most of the newer epoxy formulations have either reduced or no amine blush products...West recommends the 207 special purpose hardener, Mas 's new formulation is non-blush and so is Silver Tip....the key to getting a bond to epoxy is removal of any amine blush....scotch brite and water, wet sanding and time allowed to fully cure is a must. The other problem is the application of the gelcoat in a large area......those patch kits are great for small spots and chips but will be difficult for larger areas because if you try and bond 20-25 mils of gelcoat (less than 1/32) in one shot to the repair, it may not thoroughly dry out and cure, causing a delamination down the road.
West System recommends actually spraying thinner coats of Gelcoat to allow proper dry time between each coat. The big gelcoat manufacturers make additives for this purpose which helps thin out product for spraying...You can use one of those model/arts and crafts spraying systems for something this size...
West sells a repair DVD on this, but I have seen 10 minute versions of the DVD on-line ......I think Jamestown Dist placed it on You Tube..."how to gel coat over epoxy" which is right off the West DVD and it shows how to properly repair a 1 foot long area. You can also look up the written guide on their web site. Section 2.2.1 of their 002-550 manual.
Also, depending on how much fairing you need to do and how much room you have to "add layers" to meet the final finish height, 3M makes a structural fairing product that is Vinyl Ester based. You may be able to use that to bond and fair to the epoxy, then gel coat over the 3M product . You will still need to prep and wash the epoxy regardless. I have not personnaly used the 3M material, but have been told that will work.....
