Nigel, I have wondered the same thing, especially for mixing small quantities for touch ups. The best idea I have been given so far is to mark it off on a stick (like used for iced lollies for example, if only a small amount to be mixed) and sit it on the bottom of the container to be mixed in and fill with the appropriate component to each mark.
But how to mark a stick in such precise fractions? Their 2 part varnish is much easier at the ratio of 2 / 1. I posted the same question to the International Paints help service but they have never replied to me - perhaps they are trying to work it out too!!
I'm sure you are right for mixing up a small batch for touching up. I find it best to err on the side of too much catalyst.
International and Blakes two-part paints seem reasonably tolerant, but others I've used need to be very exact indeed. And its more difficult to get it right if the ratio is high, e.g. 10:1.
We used to have the same problem when mixing small quantities of epoxy resin for potting electronics. Found that the easiest way was to weigh the amounts. We used a chemical balance, but a letter balance would do. Weigh the container, put in about the right amount of hardener, then weigh again. Work out the necessary amount of resin, add weights to total amount, then add resin until it balances.
The difficult measurement is the hardener, because it's so little. That's why you must weigh out the hardener first.
I must say, Paul_H's solution appears to be the best way forward for those of us somewhat mathematically challenged. Why on earth International could not have formulated a simpler mixing regime than that of 2.67 parts to 1 I have no idea. Frankly, I do not have any weighing device capable of accurately weighing a couple of ounces of paint and really, for most people - or at least me! - 204 ml of hardner and 546 ml of base mixed at a ratio of 2.67 to 1 is somewhat daunting.
It's a shame that International Paints placed my query in the too hard or too trivial basket thus not worthy of a reply, I would love to have heard their suggestion. I guess thats a fact of life with some companies "customer care" services.
In case of any misunderstanding, I did not want to give the impression that I thought the quantities were not important. For example, if you used 100 ml of one, the ratio says you need 267 ml of the other - I would have thought that the round figures of 265 ml or 270 ml would have been quite accurate enough for ones measuring of it. Or if weighing or using a measure on a stick, work to sensible dimensions, not the exact odd millimetre or gram. Obviously, if one has any personal preferences to adding more of one or the other, that is an adjustment of ones own.
I think you guys are making a lot of fuss about nothing. Mark your dipstick or your measuring pot as accurately as you can and get on with it! three of one to eight of the other is easy - use spoonfuls, ladles, jam pots, whatever. If you really want to be accurate use medical type syringes like the grp people do.
The real problem is what to do when all your paint rollers dissolve and drop apart in the middle of the topsides - but it looks ok from a distance!
but if you mix the top can with the bottom can you get the right ratio.
or are you trying to mix part quantities?
also have 7 packs of grey for sale £5 a pack + post also the under coat at the same price
stu