"I have read about using a warm bath to reduce the bubbles.... am I correct that most bubbles come from the mixing of the two parts? if so why wouldn't you pour the 2 parts and place the one container in the water bath and mix while being heated."
You are correct,
All the bath is for is to reduce the viscosity (making them thinner) of the two liquids allowing the trapped gases to easily escape. My epoxy is stored in may shop and I try to maintain 66 degrees most of the time (I probably haven't mentioned that I live in Minnesota where it's now 8F degrees and feels like -10F with the wind chill) I have to turn up the heat to about 80 degrees to get the shop climatized for doing epoxy work. If I were to go to my shop now the resin and harder would be thick as molasses and would be a milky white when mixed together. The milkiness is formed by thousands and thousands of micro bubbles, which will disappear as the epoxy begins to warms and thins out. When I have the two liquids up to temperature they will be thinner and will not cloud up when mixed together.
It's a matter of preference, either will work and I've tried both. But, remember once you start to mix the resin with the harder a chemical reaction called exothermic starts and their is no way to stop it. So, if you premix, place it in a warm bath say 10 minutes you have just lost 10 minutes of working time, then you may have to account for the time you lose by heating the resin. Warm resin while start to cure faster. I haven't experienced a lot of lost time, because most of my projects are relatively small.
Here is what ArtResin says about using a bath, which I've heard from other manufactures as well.
"The working time of ArtResin epoxy resin ( also called the pot life or open time ) is about 45 minutes. This is how long you have to work with the resin before curing sets in and it becomes too stiff to manipulate. You're best to apply the resin as soon as it has been thoroughly mixed for 3 minutes.
TIP: the working time can be affected by heat: if you've put the resin in a warm water bath or if you're working in a warm environment, the working time will be decreased by about 10-15 minutes. The cure time may also be decreased."
I prefer two warm the resin and hardener separately, exothermic will not start until I start combining the two, if I get interrupted for some reason I can simply cover the glasses and rewarm them another day and of course I'll save 10 minutes of open time. I would never recommend mixing while my mixing cup was in the warm water bath, for me their is just too much of a risk of contamination, remember one drop of water could ruin the batch. I remember one summer on a hot day one bead of sweat dipped of my nose into the middle of my project and immediately I had one big fish eye to deal with. I was lucky that this happened at the beginning of the pour, so I was able to scape it off without much problem and had mixed enough epoxy to fix the area. This leads me to another tip: always mix more resin then what you think you'll need, and keep a few epoxy molds around to use up any left over epoxy.
I also prefer using glass glasses/bottles, this is because exothermic reaction creates heat, which can melt the wax off the inside of paper cups and I 've read somewhere that once heated plastic cups become somewhat porous leading to moisture contamination. I have not experienced the latter and have used plastic cups with out issue, but way take a chance when the Dollar Store sales glassware so cheap.
"Do you sand (320 grit) between pours... to help the surfaces adhere better? or to remove small surface blemishes"
. Yes and No.
If the epoxy is tacky to the touch then I don't sand, but if the resin is cured then you will need to sand it. I don't worry about the grit as much as having a light touch, so I don't sand through the epoxy and scratching the underlying finish. Epoxy is a wonderful thing, you could sand it with a wood rasp (not suggested) and then apply a flood coat and you wouldn't know the difference.
There are so many different types and uses for resins that the best advance I think I could offer is to always follow the manufacture's instructions/recommendations .
Sorry for the long winded answers
Hope I answered your questions.