Polyester is bad, Epoxy is good.
There are two main types of resin used in the surf industry, polyester and epoxy. All you need to know is : polyester is bad, epoxy is good. The information below only covers using epoxy over wood boards. Epoxy is more environmentally responsible, less toxic to work with, but most importantly, it makes a stronger, longer-lasting board.
Epoxy’s only real drawback is it is a skin sensitizer so avoid skin contact at all times. If you get it on your skin, stop and clean it off with white vinegar or a citrus based soap. Epoxy is easily cleaned from tools with denatured alcohol. I recommend wearing safety glasses to protect your eyes from splashes even though epoxy is unlikely to splash do to its thick viscosity. In the unlikely event that you get it in your eyes flush with copious amounts of water. You do not need to use a respirator with epoxy, but you should use disposable rubber gloves at all times when handling epoxy. Another precaution is to put a plastic tarp below your glassing stand to catch the resin that will drip on the floor. When the epoxy cures on the tarp, it will flake right off and it is crazy hard to remove from your garage floor.
Epoxy comes in two components: resin and hardener. There are two primary hardener speeds: fast and slow. Epoxy mixed with fast hardener cures enough to handle in 3-4 hours (the warmer the air temp, the faster the cure), epoxy mixed with slow hardener takes 5-6 hours.
Epoxy resin/hardener need to be mixed in a 2:1 ratio by volume. There are 3 standard methods of measuring epoxy ratios. Digital scales, metered resin pumps, or graduated cups. I use transparent graduated cups. The only real problem with the graduated cups is over pouring. My almost foolproof method to avoid over pouring is marking my cup with a Sharpie before I start. Part “A” is poured first into the marked cup then Part “B” is slowly added into the same cup watching as the level slowly raises to the second mark. In the event that you over pour you can easily add the required Part “A” to correct the ratio. Without the Sharpie marks on the cup the mistakes are more difficult to fix!
Another fool proof method is: First pour part “A” and Part ”B” into their own separate cup. Then combine the resin in cup 1 and mix for 1 minute before pouring the resin into cup 2 for the second minute. This might be a good method for splitting your required amount if you are working alone?
Entrophy Resin has made an excellent video tutorial on how to properly measure & mix their epoxy. This system will work with the System Three epoxy, Resin Research epoxy along with all other 2:1 epoxies.
As a rule of thumb, for the laminating stage, you want to use at least 2oz of mixed resin per foot of board length for surfboards. So for a 10-foot paddleboard, you will need AT LEAST 20oz of mixed epoxy/hardener, or 14 oz of epoxy mixed with 7 oz of hardener. This is the absolute minimum as paddle boards are wider than your standard surfboard! Beginners should plan to use a little more, like 16 oz of epoxy with 8 oz of hardener, or 24 oz total mixed material for a ten-foot board. These numbers are given as guides as the real numbers depend on how thick the resin is applied and how much is on the floor. You must use enough to wet all the glass with no dry spots. You should mix your epoxy/hardener thoroughly for at least two minutes to make sure it is completely blended. Use clean, strong mixing sticks and make sure you scrape the bottom and sides of the mixing bucket to get all of the material blended.