Woodworking Tip of the Day: Measuring Epoxy Right: Mix Ratios, Pumps and Why 'Close Enough' Fails

Pull up a stool….. this is the least glamorous step in epoxy work, and it decides whether the rest of the job lives or dies. Nobody brags about measuring. But most of the sticky, gummy, "why won't this cure" stories start right here, and WEST SYSTEM says so too: most problems with the curing of epoxy can be traced to the wrong ratio of resin and hardener. Winter is coming, the shop is cold, and I'd rather scrape nothing off the dory this year.

Why it matters: Epoxy isn't paint that dries. It's two parts that react with each other, and the reaction only finishes if the right amounts meet. An Epoxyworks article from WEST SYSTEM's technical staff puts it this way: with excess hardener, hardener molecules stay unreacted in the cured epoxy and weaken it, and they can keep later paint coats from setting up. Too little hardener is no better. WEST says too much or too little hardener affects both the cure time and how thoroughly it cures. That's why their manual says, in capital letters, not to adjust the cure time by changing the ratio. If you want it faster or slower, pick a different hardener. The ratio stays put.

Materials

• Resin and the hardener made for it (same system, same brand)
• Matching calibrated pumps, or a scale, or straight-sided graduated cups
• Clean mixing containers: plastic, metal or wax-free paper (not glass, not foam)
• Mixing sticks with a flat end
• A stiff plastic or metal scraper and a solvent for the bad-batch day
• Gloves, safety glasses and paper towels
• Your product's data sheet, because the numbers below are WEST SYSTEM's and other brands differ

Tools

• 300 Mini Pumps (or a gram scale, or a stick marked in equal parts)
• Roller pan, for coating jobs, to get the mix out of the cup
• A small test cup for the pump check

Steps

1. Read the ratio off your hardener. With WEST SYSTEM 105 Resin, the 205 Fast and 206 Slow hardeners are 5 parts resin to 1 part hardener by volume. The 207 Special Clear and 209 Extra Slow are 3 to 1. The pumps are matched to the hardener, so a 205/206 pump and a 207/209 pump aren't interchangeable, and the group size (A, B or C) has to match too. Other makers use other ratios: System Three Clear Coat is 2 to 1 by volume, and MAS LV resin is 2 to 1. Check your product's data sheet.
2. Prove the pumps once. If equal full strokes are going in, the resin and hardener containers should run empty at about the same time. WEST's pump sheet also says to mix a small test batch and watch it: it should cure to a hard solid, overnight at room temperature or warmer. Epoxyworks suggests 3 strokes each, stirred thoroughly, left until fully cured. Do it with new pumps, and again any time curing acts up.
3. One full stroke of resin for each full stroke of hardener. Push the head all the way down, then let it come all the way back up before the next stroke. WEST says partial strokes give the wrong ratio. Same number of strokes from each pump, every batch. In a cold shop the pumps move thicker material, so press slowly and give them extra time to return. Keep the resin and hardener at room temperature if you can.
4. No pumps? Measure carefully. By volume, use a container with straight sides and a square inside corner, graduated or marked with a stick, and pour the resin first. Epoxyworks points out it's easier to see the space for the hardener that way. By weight, don't use the volume numbers. The weight ratios are different because the resin and hardener have different densities. For WEST 105, the data sheets give 5.19 to 1 (205), 5.36 to 1 (206), 3.64 to 1 (207) and 3.68 to 1 (209). Per 100 g of resin, that's roughly 19.3 g, 18.7 g, 27.5 g and 27.2 g of hardener. The manual rounds 205/206 to "five parts resin to one part hardener" and 207/209 to 3.5 to 1 by weight, which sit inside the data sheets' acceptable ranges. Use the data sheet, though. That's why the plan has a table.
5. Stir at least a minute and scrape as you go. WEST's US instructions say at least 1 minute, longer in cooler temperatures. Their March 2025 manual's figure says 2 minutes. In a November shop I go with the longer one. Scrape the sides and bottom, and use the flat end of the stick to get into the inside corner. Unmixed resin hiding in the corner is how you get a soft spot. MAS Epoxies makes the same point about soft spots, and adds a pouring tip: don't scrape the last drips and film off the walls of the cup onto your work, since that's the part that might not be mixed.
6. Thicken after, never before. WEST's manual calls it a two-step process: mix the resin and hardener thoroughly first, then blend in the filler in small amounts. Their problem solver says the same thing: only add fillers or additives after the resin and hardener have been thoroughly mixed.
7. If it's still tacky when it should be hard, think before you recoat. If the shop is cold, epoxy cures slower, so allow extra time and add gentle heat (WEST warns that unvented kerosene or propane heaters can inhibit the cure and contaminate the surface). If it was off ratio or under-mixed, waiting won't fix it.
8. Remove the bad batch. WEST says don't apply more epoxy over non-curing epoxy. Warm it to lower its viscosity and scrape off as much as you can with a stiff metal or plastic scraper. Clean the residue with a solvent (WEST's current manual names its 850 Cleaning Solvent), with plenty of ventilation. Let the solvent dry before you recoat, then recoat with a fresh batch.
9. Find out why before you mix again. Equal full strokes? The right pump for the hardener (5:1 or 3:1) and the right group size? Pump ratio checked? Resin and hardener from the same system? WEST's list also includes wrong products: resin won't cure properly with other brands of hardener.

MOAM's shop note: "Close enough" has a real meaning, and it isn't eyeballing. WEST's data sheets publish an acceptable ratio range by weight for each hardener (4.83 to 6.20 to 1 for 105/205, for example), and it's wider on the resin-rich side. Calibrated pumps used with full, equal strokes keep you comfortably in it. A cup poured by eye, a half-pumped stroke, or a "splash more hardener to speed it up" is how you end up outside it. If you slip, err toward a little extra resin, not extra hardener. Epoxyworks says the same about a pump that spits air. Then write the hardener, ratio and pump strokes on the shop wall once, and stop doing the math on the fly.

Sources: WEST SYSTEM, "Dispensing and Mixing" and "Epoxy Problem Solver" (westsystem.com); WEST SYSTEM User Manual & Product Guide (2020 US edition and March 2025 edition); WEST SYSTEM 300 Mini Pump Instructions; WEST SYSTEM data sheets for 105/205, 105/206, 105/207 and 105/209; Epoxyworks, "Getting the Most out of Mini Pumps" (Jim Derck, GBI, 1995); System Three, Clear Coat product page and data sheet; MAS Epoxies, "The Ultimate Guide for Mixing Epoxy Resin" and 2:1 LV resin page; Entropy Resins technical video, "Mix Ratio: Measuring and Mixing Epoxy". Ratios and mixing times differ by product, so check your own data sheet.

MOAM Out.