I really like and have used Spectrum gel coat paste available from Catalina Direct. It is factory matched to the gel coat used in your year of manufacture. On my boat the match is perfect. C Direct price is better than from the factory by a lot. Read about using the product. It has no shelf life. Once opened it should all be used in a week. I usually mix a quarter jar at a time. 
I grind out the crack or chip wth a dremmel tool and then clean with acetone just prior to filling. Fill a little proud of the surface as the filler seems th shrink a little while hardening. Sand down starting at 180 or 220 then keep going finer. 320 is usually as far as I go

Like most catalyzed products, this stuff is temperature sensitive. You can get around this with careful use of a heat gun but that can be tricky. Go to the Spectrum website for detailed instructions and temp limitations. Spectrum says that patches can be sanded fairly soon after application. I have found that a 24 hour cure is better. I don't know if this has to do with temp but it probably does.
One thing that should be made loud and clear, the catalyst used in this and many products is potentially super damaging to eye tissue, permanent damage in seconds, so when adding the drops to the mix take normal precautions and mix well away from your eyes.

Agree on the Spectrum factory matched colors being a big plus.  I had some color problems and they took the can back, tested it and found it was off and replaced it.  The white blended perfectly straight out of the can.

I have found around 70 degrees to be ideal for application.  Lower is okay but your cure times go up..  I have used at higher temps but had to keep the unmixed resin in an ice chest to slow the gel time.  Even with 70 degrees you are talking 10 minutes to apply.  

For smooth deck areas buy a couple of Preval sprayers at Home Depot they work great for spraying.  The pro's use them for small repairs.  Also get a pint or so of PolyVinyl alcohol.  It is inexpensive it you get it online or at a mold making shop (used a lot for mold casting in theater, ceramics, etc.)  I think I got a quart for $12 that way.  After you spray several layers of gelcoat, spray it with PVA to cure, then come back next day and wash off with water.  It is so easy.

When you order gelcoat also order some Cabosil (thickener) and Styrene.  You probably cannot find it locally.  If you ask a boat shop they will probably tell you to use Acetone.  I recommend that you don't do it unless you have to.  Acetone will thin gecoa​t and when you spray most will evaporate but any left in the resin during cure will weaken the gelcoat.  Styrene on the other hand chemically becomes part of the cure and does not weaken the gelcoat.  Like I said most repair shops don't bother with this and styrene is nasty stuff but so is Acetone.

Use a good respirator with organic vapor cartridges.  

Lastly, don't use epoxy in your repair scheme unless you are prepared to put a tie coat down between the epoxy and gelcoat.  Gelcoat will flake off epoxy as the amines coming off the epoxy attack the gelcoat.  Just use fiberglass resin if you have to repair deep into a crack then sand smooth, fair and spray several coats of gel to get about 30 mils.