In my upcoming list of projects, I have a few ideas on the design table that involve using polyester casting resin. Before I get into polyester resin tutorials though, I want to talk about several things you need to know before you use polyester casting resin.
1. The smell is horrible. Unfortunately, this fact is not clearly stated on polyester casting resin containers. I admit that my nose might be a bit sensitive to smells, but I have read the same comments from other resin casters as well. For me, I will NEVER use this resin inside my house or office. I will use it inside an enclosed space (like a garage) if I can leave the space, close the doors, and not reenter for twelve hours or more. When working with this resin, I will use a NIOSH approved respirator for fumes. *Note: these are not required according to the safety information of the polyester resins I use, but I find the smell bothers me that much. If didn’t have a NIOSH approved respirator, I would only use this resin outside. In addition to that, I would have a fan blowing fresh air on me or evacuating air away from the casting. Yes, I realize this is overkill, but at least to me, the smell is that bad.
2. This resin is not for beginners. It requires more skill in several respects. First, the pot time is shorter than many epoxies, generally 8 minutes or less. Note: The polyester resins I have used state their pot time is not that short. Maybe it is because I’m in a warm weather climate, but none of the polyester casting resins I have used have ever gone more than 10 minutes before starting to gel. To make things even harder, polyester casting resin does not mix in a 1:1 or 2:1 formula, but instead requires drops of hardener per ounce of resin. If you are struggling to get a good mix with larger volumes of hardener mixed into the resin, you are not going to be successful only mixing in a drop or two of hardener into the resin.
3. You have to start with the end in mind. While you can mix a volume of epoxy resin and hardener and use as you please, polyester casting resin is not that simple. The amount of hardener you add to polyester casting resin is dependent on the depth of the final casting. It seems counterintuitive, but the thinner the casting, the more hardener is needed for curing to occur. This is because thin layers are unlikely to generate enough heat to cure. You will need extra hardener to get curing to take place. To take that one step further, when pouring multiple layers of polyester casting resin, each layer has an additive heat effect. What this means for you is that subsequent layers likely need a bit less hardener if you are casting onto a layer that hasn’t fully cured yet. What happens if you do not reduce the amount of hardener? The resin can heat up to quickly and cause cracking around your embedments.
4. The shelf life on polyester casting resin is no more than 6 months. While many epoxies will work for years if properly stored (although the hardener may start to yellow), 6 months is generally the longest you can expect polyester casting resin to stay in usable condition. Note: In troubleshooting problems for readers who purchased their polyester casting resin from a ‘big box store’, oftentimes, I find the resin was likely purchased after the 6 month shelf life. I make sure Resin Obsession rotates stock regularly to give customers the longest shelf life possible for all of our products, not just polyester casting resin.
5. The surface exposed to air during curing will remain sticky, even after the resin has cured. You will either need to sand off this surface or seal it with a layer of resin gloss sealer spray to cover the stickiness.
So with all these disadvantages to using polyester casting resin, why should you consider it?
Polyester resin castings cure very hard. They cure hard enough that you can use a buffing wheel and compound to polish polyester resin castings. That means you do not have to recoat with another layer of resin or a gloss spray to get a shiny surface. A few seconds on a buffing wheel is all it takes to get a bright, shiny finish. That being said, however, if a polyester casting is dropped on a hard surface, it is likely to break.
Polyester resin is also generally ‘moisture insensitive’. Your environment’s humidity generally does not affect its performance.
What has been your experience with polyester casting resin?