This is an issue which requires some forethought of how much epoxy “babysitting” you wish to do. Although one “Casting” pour is often convenient there are many advantages to multiple single layers. The first being cost. Monomer catalysts are significantly different in casting epoxy causing cost to often exceed double the expense of single layer epoxy materials. If something goes wrong with a casting pour such as improper curing or becoming a landing pad for a few local flys, you live with the submerged terrorists in your composition, must wait for epoxy to fully cure before your can drill out and fill, or are just plain out the cost of materials to try again. However, with a single layer application, you can scrape off the compromised material, sand the previous layer and continue on with the usual process. Second, cured casting epoxy is usually softer than single layer epoxy. Therefore some people will opt to pour a single harder coat top layer over a casting pour to afford additional durability. This however imposes a need to obviously purchase two different kinds of epoxy anyway. While on the topic of durability, it can also be understood that there is "Strength in Numbers." Like steel cables, ropes, and cloth have many single strands of material woven together to afford multiples of structural integrity, the same can be understood for layering epoxy. Another benefit of the layering process is that it creates good depth perception for semi translucent pours. The first bottom layer starts off opaque and each subsequent layer gradually becomes more translucent. Next is mitigation of air bubbles. Air bubbles are able to be eliminated much easier from ¼ “ of epoxy than with 2” of surface tension over them. Finally, the fact is that while casting epoxy needs a minimum of 48 hours to cure, you could pour a single ¼” layer every six hours on top of a previous layer. Therefore, at the end of two days, both methods filled the same amount of space but the single layer method just required more elbow grease.
Time and efficiency is obviously the big factor in choice between the two options. A person doing commission work having multiple epoxy projects going at once may not find themselves too inconvenienced in the layering method as they are routinely handling epoxy material anyway. The lower material cost also helps commission workers to remain competitive by recognizing better profit margins on their work. The main issue at hand is where the builder may be on the learning curve and what they may be able to trade off between quality, time, and cost.