Measuring Very Small Amounts of Catalyst

Finishers use all kinds of smart methods, including weighing with a scale, to measure and mix very small batches of catalyzed finish. July 7, 2011

What kind of measuring cups are you using when you have to measure out a small amount of catalyst? Sometimes when you need an extra 8 oz. or so to finish spraying a job, and your finish only requires about 5 oz. of catalyst per gallon, measuring precisely can be tricky. I haven't found any measuring cups that read in ounces that small. Should I convert it to mls and use those droppers you can get at a pharmacy?

Forum Responses
(Finishing Forum)
From contributor J:
Get a digital scale and do it by weight. Much more flexible, much more precise.

From contributor S:
Do all the suppliers have the amounts of catalyst required in grams?

From the original questioner:
I like that idea of using a digital scale. How do you do the conversion from ounces to grams?

From contributor J:
I don't know the answer to contributor S's question, but it doesn't take much math to figure it out if they don't.

From contributor T:
1 ounce = 28.3495231 grams
My digital scale measures in ounces or grams.

From contributor J:
1 oz. = 28.3495 grams.

If you Google "ounces grams" (or any other combination of units) you'll turn up all sorts of charts and conversion calculators.

From the original questioner:
Thanks for all the help. I just found an app on my phone that will do the conversions.

From contributor D:
Be careful though. I may be wrong, but I believe most catalysts from the manufacturer have the directions given for volume mixing. Fluid ounces (volume) will not convert directly to grams (mass) unless you know the density of the catalyst. You will need to know precisely the density in g/mL of the catalyst to be able to figure out the weight of catalyst needed.

From contributor J:
Contributor D makes a good point. If the manufacturer gives you amounts by volume only, then you'd need to weigh known volumes of the finish and catalyst and adjust accordingly. It can be a bit fussy to calculate the ratio the first time around but you only need to do it once for a given finish, and it's very helpful to have this tool in your pocket. Another helpful tip: keep the scale in a Ziploc bag, even when you're using it.

From the original questioner:
Good tip about the Ziploc baggy. I think as soon as I get the conversions, I will make a quick reference chart to hang on the wall in the spray room. I will probably put it in 8oz. increments, so nobody has to stop and do math while in the middle of finishing a project.

From Paul Snyder, forum technical advisor:
Some brands use a mix by weight ratio and some use a mix by volume ratio. To mix small amounts of catalyzed finish, I use the 2 ounce touch-up bottles from E-Z Mix. The small bottles are marked in 1/8 oz. increments as well as the graduated scales for various ratios of catalyst and reducer. You get 50 of the bottles per box.

From contributor J:
You don't need to spend hundreds of dollars on a high-end scale, but you should be careful to choose one that reads in sufficiently fine increments. I have a kitchen scale that reads in grams when set to metric, but only tenths of an ounce in imperial; the metric measurements are 3X finer even though the same sensor is being used for both.

You'll find that scales are sort of like LCD computer monitors, in that there are just a few common capacities because the sensors are fairly standardized. A 5000g/11 pound scale is a good size to have. Keep spare batteries on hand (or use an AC adapter), because these scales do odd things when the power gets low.

From the original questioner:
I found a digital scale at Wal Mart for around $20-$30. It reads in grams. Would that type of scale be accurate enough?

From contributor J:

It should be, assuming it reads in single grams. I say this because some read in grams but only in 5g increments.

From the original questioner:
If I can't find the numbers I'm looking for to convert the catalyst from fluid oz (volume) to grams (mass), could I just take a gallon of catalyst (say 5.5 oz.) and weigh it on a digital scale? Then weigh it in grams, then break it down from there so I know how much it should weigh for smaller amounts of finish?

From contributor J:
Absolutely. Since you want to make a chart to hang on the wall, I'd just put together a quick spreadsheet to do all the calculations for you. If you're comfortable with Excel, it will only take a couple of minutes. Perhaps obviously, you'd also need to weigh the gallon of finish (minus the can).

From contributor M:
SW sells a 2.5 oz paper specimen cup for about $10 for a sleeve of 100. They are graduated to hand almost any ratio you may need.

From contributor E:
Weighing finish on a digital scale is great. Once I started doing this I never wanted to go back to volume. I use the PPS cups and measure it right in the cup I spray out of. Be wary of cheap scales, though. You want a scale that will measure to 1/10th of a gram (0.1 decimal point). The more accurate the scale is, the smaller amounts you can measure accurately.

Some tips for using them:

Keep air flow away from the scales; shut off nearby fans, booths, etc. My scale is so sensitive that if you blow at it from three feet away, it will jump up and down several decimal points.

Keep it on a solid vibration free surface; don't hammer paint cans closed on the same table as the scale.

Keep them clean; wipe up drips as they happen.

Any good scale should come with a calibration weight and you should recalibrate after moving or unplugging the scales.

The weight of catalyst will vary among brands, but yes, you can weigh a known volume and then divide by the number of ounces. I like weighing larger amounts if possible to minimize visual measuring mistakes. For instance: you will probably get better accuracy measuring a full gallon then dividing by 128 than trying to measure 1 fluid ounce by eye in a measuring cup and weighing that.

From contributor I:
I use a 20ml syringe for ML Campbell Aguabarnice catalyst.

From contributor C:
I use paint cups from any auto paint store. You can get them down to 1/2 pint size. They have several scale markings on them. For smaller mixes, I use a 2 oz shot glass. Sold in liquor stores, they have several scales on them too.

For even smaller mixes, I use plastic pill cups. I think they go up to 1/2 oz. Real handy for my air brush mixes.

From contributor W:
Can you simply mix 1 oz of catalyst to 10 oz of thinner? Now every oz of this mixture becomes 1/10th oz of catalyst?

From contributor C:
That would have to be 9 oz thinner to 1 oz to have a 1:10 ratio. I guess that would work.

From contributor V:
I have an assortment of syringes that I use for measuring additives and thinners. I don't even use the plunger. I have my additives in squeeze bottles and pour directly into the top of the syringe. I have mixing charts (Excel) above my mixing station that has both ounces and milliliters (cc's). You can get the syringes online or at the local pharmacy. They're inexpensive and accurate.

From contributor Z:
I think you could have a serious problem if you thin the catalyst before mixing into your finish. The data sheet for a 2k urethane I use specifically cautions against doing that. Mix the catalyst and finish first - then thin as needed. I like the Tri-Pour beakers from US plastic for mixing small quantities. They are inexpensive - graduated in ml and oz.