The additive they use is called 'Patchaid', made by a company called
'Spectrum'. They said they use to use something called 'Dura-tec', but
the 'patchaid' worked better. That and the hardener is all they use
with un-waxed gelcoat. 30 to 50% additive is used depending on which
gun they were using, whatever it takes to make it sprayable, on small
repairs they would use one of those disposable sprayers, 'Prevail' I
think it's called, which you can get at Home Despot for about $7. It's
still not a glossy surface like you get with paint or Imron, they have
to wet sand and buff but it cuts down on that a bunch. I looked on the
can and all it said was 'contains styrene monemer' so it wouldn't
surprise me if all it was was styrene and wax for 10 times the price.
Like Harry says, the boat dealers can get you the correct color,
whether it matches or not sort of depends on the age of the boat and
the color you are trying to match, lighter being easier. On most boats
if the color match is critical, they have somebody come out with a
'gun' that reads the color and and formulates a 2-part paint that they
apply and then cover with a clearcoat which gives them a pretty much
invisible patch.