Best practices when using urethane casting resin
Mixing Tips: Once each side is measured and weighed in separate clean plastic containers, pour both A and B into a third clean plastic container for mixing. **Pour immediately after mixing!**
Never mix less than about 3 ounces of product: When manufacturers design and test their resins, they normally write the specifications for 100 gram batches, which is about 3 ounces.
There are two bad things that can happen when mixing a smaller batch. 1) Because the sample is small, it is much more difficult to get the mix ratio correct. 2) These mixtures are exothermic, meaning that they generate heat in order to cure. A tiny batch does not generate enough heat to cure the resin properly.
Avoid mixing with drill motors: Mixing with an electric drill can cause a few problems. They don’t get into every corner of the mixing container. Also, if they spin too fast, they can generate friction in the resin causing it to exotherm out of control resulting in premature curing. Powered mixing also can generate a lot of air bubbles.
Mold Release: If you use a mold release, let it dry for a while. A spray can of mold release contains a lot of solvents and propellants. These compounds need to evaporate off the surface so they don't cause bubbles. Check the dry time of the mold release from the directions on the label.
Do not vary the mix ratio: Unlike some polyester resins, altering the mix ratio to vary the cure cycle does not work with urethanes.
Coloring our white casting resin: Add our urethane colorant by drop until you get the color desired. NOTE: You will NOT get black or red with our white casting resin. For black, use our jet black casting resin. For red, use our crystal clear casting resin with our red urethane colorant.
Removing surface oil before painting white casting resin: De-mold the fully cured casting. Put the casting into talc for 24 to 36 hours. Remove it from the talc, wipe off talc and wash casting with mild dish soap like Ivory liquid soap and water. Dry the casting thoroughly. Use a high quality priming paint like Krylon Fusion or a clear automotive priming paint before your first color coat.
AeroMarine Products' fillers with casting resin: AeroMarine Products' white microspheres will make casting resin float, while our bronze powder will make it heavier.
Mix everything twice: Mix the two components together very thoroughly in a clean platic container, then transfer the mixture to another clean platic container and mix them again. The theory is that the liquids clinging to the sides and bottom of the containers don’t get mixed well. By transferring the mixture to another clean plastic container, you are assured that everything is well mixed. Any unmixed material stays in the first container.
PLEASE NOTE: When using fast, water-thin casting resins, you may only have time to mix it once. Since it is water thin, it will probably mix fine in one mix.
Mix in plastic containers: Always mix your urethanes in a clean plastic container, plastic or metal mixing tools. Paper cups and wood contain moisture which may adversely affect the polyurethane. Avoid waxed paper cups because the wax may melt and contaminate the resin.
How to avoid air bubbles: Air bubbles in urethanes are almost always caused by moisture. Do everything possible to avoid moisture getting into the mix. This includes replacing the lids onto the containers promptly after use as well as avoiding using the product during rainy days or times of high humidity. Do not pour against an unsealed water based product such as plaster or hydrocal. Seal plaster or hydrocal with something like Krylon Clear Acrylic. **We stock an aerosol nitrogen blanket called Extend-It that can increase the shelf life of the urethane during storage.
Do not mix a large batch: The larger the batch, the more exotherm or heat is generated in the cure cycle. If you are casting a large part, mix small batches to make the process more manageable. Thickness of the pour also affects the exotherm and cure speed. A very thin pour will take much longer to cure than a thick pour.
Shake or stir very well before use: The liquid components may settle in the containers during storage. Vigorously shake or stir the components separately before mixing. Let it sit a few minutes to let any bubbles rise to the surface after shaking the container.
Test: Always run a test determine the feasibility of your process. There are many unforeseen factors that can affect the outcome of your project. Running a controlled test may be inconvenient, but it can make the “Learning Curve” of processing these products much easier.