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Mixing by Weight

I normally use the increments on regular mixing cups to get my mixing ratios correct. However, I'm going to be mixing a very little amount of 2K regular build primer (4:1), less than any of my increments on the cups support. While this may seem obvious, can I just mix by weight? I'm concerned for the difference of mixing by volume, which I think is how it is using the mixing cup increments vice weight due to density differences of the two substances. Or maybe the ratio really is based on weight and I'm just getting close enough with the mixing cup increments?
Either way, is mixing by just weight okay?

Barry does not give out that info. I'd love to have it for every product also, but we just have to deal with things as they are. Whatever you do DON'T simply mix by weight based on the ratio, because the product and the activator have different densities. What you can do, is if you ever mix a large amount, you can record the weight of the product vs the weight of the activator, and then possibly use that as your actual ratio. For instance, I know that the activator for Euro is denser than the clear, so the one part of activator weighs more than one-forth of the required amount of clear. For primer, the situation is no doubt reversed, but I have not measured any primers that way.


Top Banana
I keep glass jars food comes in of all sizes with metal lid & old prescription bottles for very small amounts.
Today i wanted enough uvc to clear 2 taillights. Used narrow jar, put tape on side Before adding water.
Used tiny measuring glass, put 20ml in, marked paint jar, 20 more & marked jar again. Idiot proif heh.

I do this with marine 3:1 epoxy and everything else. Used tomato sauce jars when i painted my cars.

Didn't answer your question but it works : )
Don't ever mix by weight. You will be so wrong it won't be funny. I stress with the ones I help and train to always use a flat bottom straight sided container, a new pint or quart paint can works great, and a paint measuring stick (PPG still has them last I checked). I don't like the mixing cups you get from the jobbers I have gotten bad measurements from them so for me one bad measurement and I don't use it and discourage others too.
The measuring sticks are alum. and have measurement on them in a couple different ratios to match what you need.


Here are some that I have. I grab extras in case someone walks off with one, I keep them next to my paint guns so I know where they are.
I don't understand why everyone is so against mixing by weight, I've been doing it for years including spi. I only do it for small quantities as I only stock qt mixing cups.
I don't understand why everyone is so against mixing by weight, I've been doing it for years including spi. I only do it for small quantities as I only stock qt mixing cups.
It’s doable as long as you take the time and weight individual activators and do the right math. I did it myself for a couple of years when I was doing mobile work and spraying bumpers all day long, it was worth it then when doing small touch ups.
i mix the epoxy by weight and when i do flattened universal. i will usually mix a pint then weigh each component then plug that into my wanda software as a custom mix. it just gives me the correct mix for the amount i want
Some people are more comfortable with the arithmetic than others. I always use a calculator but you also have to be able to compare the answer with a mental estimate to make sure a key wasn't pressed wrong or a decimal was in the wrong place. If you have a lot on your mind or haven't had enough sleep, it's easier to make a disastrous mistake using arithmetic than by just using lines on a cup. Ask me how I know, haha. If anyone has software like Jim, that's a great option. I had that from my previous paint co. but not my new one, I am too cheap to pay for it.
I understand there's more room for error and the person doing it must understand that EVERY component may have a different weight. For instance not all hardeners within the same product weigh the same. But if you need a couple of oz's of something, to me weight is the only way to go. I guess I've had mixing systems for 20+ years so I've gotten accustomed to weighing even clears and primers. I only mix clear and primer by volume when mixing more than 6-8 oz's. I've found those printed qt mixing cups far off below 5-6 ozs.
really can't understand the hate against weight. For small amounts ill typically weigh an equal volume of each product to get their relative densities and then select whatever is needed.
Well, this escalated quickly. OP here. So, I'm a pretty smart guy...operated a nuclear powered submarine for 24 years in the US Navy. If I pour out an equal amount of paint and activator by volume, weigh them, and then apply that correction factor to the ratio, it'll work? That's what I understand rlboinski's post to say. I can handle that math. I'm talking about spraying only a few oz of paint here...it's a single door for a Triumph Spitfire...I've mailed letters bigger than it. Maybe 2-3 oz for 3 coats based on my epoxy experience.
Okay, enough of me being a smart-ass...seriously, though, can I do that? Will it scale for larger amounts?

Thanks, honestly, everyone for the lively discussion. I'm learning so much it's scary!

The answer is yes, BUT... There's always a "but," right? To achieve recorded accuracy with small amounts, it's much better to first weigh a large batch as a reference, because with large pours, the percentage of error from under or over-pouring will be less.