Re: Looking to paint my 14' fiberglass boat, trying to take the cheapest route.
No, don't add the hardener at the paint store! Add it 30 minutes before you are ready to spray (mixing often during that 30 minutes). Don't mix more than you will use in an 8 hour time period.
Go to your nearest Tractor Supply store and go to their paint aisle. You can get both red and white (it's Valspar tractor/implement paint). Get a bottle or two of the Enamel Hardener, which they sell right there next to the paint.
Next, pick up some primer. I got mine from Lowes, so I'm not sure if there will be some right there with the Valspar at TSC.
For prep, use 60 grit paper to get it down to a good scuff. If you need to do any body filling/repairs, do it now (3M Marine filler is good stuff. Be sure to sand it with 60 again if needs be) Then hit it with some ~150 and finally some ~220. Wipe it down good with acetone.
Now you will spray a coat of primer. Let it dry and then lightly hit it with some ~220 grit paper to knock it down. Wipe it down with acetone again and spray a second coat of primer.
Hit it again with the 220 and the Acetone and now you are ready for color! Mix up your paint and hardener about 30 minutes before you are ready to spray. Mix it well over those 30 minutes to get it activated. Spray the color in thin coats and wait 8 hours or so between coats. A light knock-down with the 220 and an acetone wipe between coats should be all that is needed.
When you have enough coats of color, you can wetsand it, polish it, and wax it to your content.
Disclaimer - I may have missed a spot or two. WoG will correct me if I missed anything or made a mistake.