Resin mixing is an important part of composite manufacturing. Here's how to do it safely and with an acceptable margin of accuracy.

WARNING - THESE INSTRUCTIONS INVOLVE THE USE OF METHYL ETHYL KEYTONE PEROXIDE (MEK-P), A CORROSIVE COMPOUND THAT IS MILDLY "EXPLOSIVE." GLOVES ARE A MUST AROUND THESE COMPOUNDS AS THEY WILL ATTACK ORGANIC LIFE FORMS - THAT MEANS YOU, YOU MEAT PERSON!

IF YOUR SKIN IS SPLASHED, IMMEDIATELY WASH WITH SOAP AND WATER - DO NO DILUTE WITH ACETONE. SEEK MEDICAL ATTENTION IF YOU HAVE A SEVERE REACTION OR DEVELOP A FEVER.

FIRE ADVISORY - POLYESTER RESIN IS FLAMMABLE. DO NOT SMOKE OR HAVE AN OPEN FLAME NEAR YOUR WORKING OR STORAGE AREA. DO NOT LEAVE CATALYZED RESIN UNATTENDED WHEN IN SIGNIFICANT QUANTITIES AS THERE IS A POSSIBILITY OF SPONTANEOUS COMBUSTION. SHOULD YOU HAVE A FIRE EMERGENCY, SMOTHER THE FLAME.

The following video is long, but full of tips, tricks and advice for working with resin for layup.

Glossary
MEKP - Methyl Ethyl Keytone Peroxide - polyester resin catalyst (hardener)
PVA - Polyvinyl Alcohol (film mold release)
Cabosil - the white silica filler being mixed in (trade name - aka Aerosil)
Tool - negative mold used to make parts
Micro Balloons - air filler encapsulated in silica glass (creates a lattice matrix structure in resin)
Gelcoat - provides a high quality finish to composite tool surface
Composite - material made from two or more substances (in this case fiber glass and resin)
Mat - Reference to chopped fiberglass in a sheet form; tearable by hand with strands about 1.5" long
Exotherm - reference to the heat generated/required by the resin to cure
Pot - Catalyzed batch of resin
Pot Life - Working/usable life of pot. After this time, the chemical reactions between catalyst and resin "take off"

Step 1: What Is It?

Polyester resin is a synthetic resin (that is, it starts off as a fluid and hardens). Polyester resin is one of the more common forms of resins - epoxy resin being another very common form. When it comes to the marine industry, polyester resin is the most common as it is more rigid than it's epoxy counterpart. Keep in mind, the trade off for it's higher stiffness is that it is more brittle.

In its viscous form, polyester resin is flammable and a skin irritant - something we expect from a styrene based fluid. Manufactures mix their own blend of cobalt and other conditioner additives to help the curing process - however, Methyl Ethyl Keytone Peroxide is used as a catalyst.

Polyester resin is very common in tooling - that is, making molds (typically negative molds) that will be used to make a product. A major reason for this is, again, it's rigidity.

Finally, polyester resins are compatible with any type of fiberglass, fibrous carbon, Kevlar and urethane type foams (NOT styrene type foams). While the resin itself is compatible, the catalyst may not be (especially with foams). A test sample is always recommended before tooling.

Step 2: Where to Get It?

Almost any marine supplier will carry it. As well as several other online suppliers. I have used a company called US Composites. As my experience with them has been excellent, I will return the service with a link to their product.

US Composites

Step 3: Mixing - No Fillers

Mixing without a filler or additive is actually quite easy. Simple measure off the amount needed and, when you're ready, add your MEASURED catalyst. Then mix throughly.

Here's a tip: Resin curing is an exothermic reaction - that is, the resin cures when it gets warm/hot. This heat comes from the aforementioned conditioners reacting with your catalyst. So, vigorous mixing is not necessarily recommended as the additional heat will reduce your pot time - this goes double for thickened resins.

Here's Another Tip: Pouring Resin from large 5 gallon jugs is a pain in the butt. Unless you know how! Those spout tops are positioned such that if you tip it on it's side with the top at the higher side, the resin will just barely reach that level. Then, roll the can to pour resin. See video for an example.

Step 4: Mixing - With Fillers

Sometimes, it is beneficial to add a filler/thickening agent. Types of thickening agents and their uses are beyond the scope of this instructable - but in general, mixing them into your resin is the same process.

1. Measure out how much resin you need.
2. Add a small amount of filler
3. Mix in well
4. Repeat steps 2-4 until the desired amount has been added OR until you reach your desired viscosity
5. Add a measured amount of catalyst (as per the resin requirements).

Remember, adding fillers can increase the amount of heat generated due to the internal friction of the resin mixture - be aware of this as it may play a role in your pot life (how long it takes before the resin starts heating up/curing).