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TOPIC: How Fast do You Shake Yours? A Thought Experiment

How Fast do You Shake Yours? A Thought Experiment 4 years 5 days ago #32392

How Fast do You Shake Yours? A Thought Experiment
- or, Aerosol Shaking: Is Less More? -


Recently, I was prompted to think about my technique for paint aerosol agitation. Watching one of the many, many 1:1 car restoration shows on UK free-to-air TV, I saw someone shaking a rattle can in a rather desultory fashion. Was this simply a case (Occam's razor and all that) of a lazy mechanic, or did he know something I didn't?
I decided to try a more relaxed method of paint agitation, and you know what, I think I actually got better results - denser coats, especially from the larger (400ml) cans of primer & Satin black.
The question is, is there a scientific explanation as to why less is more?

First, let's have a look at the instructions for shaking, from a number of manufacturers:
• "Shake well before use" (Humbrol Hobby Spray)
• "Shake thoroughly before use" (Hycote 250ml paints)
• "Shake can vigorously before use and reshake frequently" (Tamiya PS paints)
• "Shake can vigorously for one minute before spraying and occasionally while in use" (Mean Machine Chrome)
• "Shake can vigorously for at least two minutes" (Hycote 400m l paints)
• "Shake the can vigorously for 2 minutes once the agitator ball is heard to rattle" (Halfords)

No agreement there then - but the word "vigorously" does appear a lot, and the longest time mentioned is two minutes, so we'll take that as the gold standard - though I don't think I'd want to try that on the Mean Machine Chrome paint, due to its two unusually large balls.



Shaking vertically as fast as possible (surely that's the interpretation of "vigorously" that most people would use? It's certainly my usual method) resulted in a hand speed of 344 strokes per minute for the first 15 seconds, dropping to half that rate (172 strokes per minute), so a theoretical maximum of 387 strokes in the recommended two minutes.

However, I don't find movement at that speed natural & it does build up tension in the arm just when you need to be relaxed for better nozzle control - and the total number of strokes is probably less than that as muscle tension increases. This "fast toss" technique is probably great for bicep development - do it several times a day & you'll end up with a right arm like Arnold Schwarzenegger, but unless you can use your other hand with equal dexterity you'll end up with an asymmetric look that'll be familiar to ten pin bowlers and fans of the Cindy Crawford Workout video circa 1992.

Slowing down to the minimum I found comfortable showed a rate of 86 strokes per minute, easily achievable for the full two minutes, with either hand. I did try going slower, but found I just couldn't get to anything approaching 60spm on a repeatable basis, I just kept speeding up.

One possible explanation for the improved results of the "slow toss" method is stroke length - with the fast toss you're already having to brake ready to change direction immediately after starting the upward (or downward) stroke, this builds tension in the arm and also results in a very short stroke - I observed a typical distance of only 50mm to 100mm (2 to 4 inches).

With the "slow toss" however, the stroke length increased massively, the distances observed being 300mm to 500mm.

Multiplying up the rate by the stroke length showed that the total distance the can travelled in two minutes shaking was 19.35 to 38.4 metres for the fast toss; trumped convincingly by the slow toss, where the distance travelled was 51.6m to 86m.


These headline numbers are possibly misleading, as it's obvious that distance travelled is not the only factor - dropping a can on a bungee of a length appropriate for a 86 metre round trip would not result in paint mixed to the same degree.

What I suspect is going on is that as long as the stroke length is at least as long as the free volume can length (or some percentage over) - 155mm in my experiments, then the results will be better than a method of shaking where the stroke length is less than the can length - i.e. a full stroke is better that a partial one.


Proper testing (at least for proof of concept) would need to be repeatable & ensure as much consistency as possible, so I'm thinking several cans of the "heaviest" paint (probably red primer?) from the same batch handled the same, shaken on a mechanical arm that can be set for varying stroke rates & stroke length, then a second mechanical hand to ensure even pressure exerted on the nozzle and sideways speed, then measure then density of the results, possibly by weighing the results, maybe a more high tech approach of quantifying the opacity (something like a tint meter). To be honest though, I'm quite happy with the results I've seen and the conclusions I've reached, so you'll have make do with my anecdotal evidence.


However, this does have some implications for my plan to make a paint shaker - firstly, that I don't need it to be particularly fast (which has a positive impact of robustness and longevity) as long as I can achieve a stroke rate on the close order of 75 to 90 per minute it'll work fine. Secondly, stroke length is important - I need to achieve a reciprocating motion throw of at least as much as the longest can I'd potentially use.

Thirdly - and perhaps most importantly - having confirmed that less is more in the paint shaking department, it'll become a lot less tedious, and I can then question whether I actually _need_ to make a machine to do something for me that's just become less of a chore.


Finally, to anyone that thinks I have too much time on my hands, I say an hour spent thinking about this was not an hour wasted ;)
The following user(s) Liked this: larbut, Manotas, stingray-63

How Fast do You Shake Yours? A Thought Experiment 4 years 5 days ago #32397

Fantastic post! :laugh:

Have you tried rotational movement? :ohmy:
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How Fast do You Shake Yours? A Thought Experiment 4 years 4 days ago #32409

larbut wrote: Have you tried rotational movement? :ohmy:



If you can't fix it with a hammer, you've got an electrical problem mate :)
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How Fast do You Shake Yours? A Thought Experiment 4 years 4 days ago #32410

There's always someone with a dirty mind :whistle: ;)

How Fast do You Shake Yours? A Thought Experiment 4 years 4 days ago #32420

I use a "vibrating" plate to mix paints.... :dry:

Cheers, Bram
Restoring Countach 58005
Restoring 58015 RR
Restoring 58098 F40
Restoring King Cab and Monsterracer
Restoring Audi Quattro rally
Restoring Mk.1 Sand Scorcher
Restoring Porsche 936

How Fast do You Shake Yours? A Thought Experiment 4 years 4 days ago #32422

Regarding stroke length: I would imagine that the optimal value would be just long enough to allow the paint and the ball bearing to travel the length of the can. Any longer and the paint is just sitting at one end of the can held under acceleration and not being agitated.
Thinking about it, it's probably the acceleration at the ends of the stroke that is important rather than the stroke length. Plus the number of strokes of course.

If you want to make a mega paint shaker I have a broken pressure washer with a working Briggs and Stratton chinese copy petrol engine you can have for free.

Reminds me of a promotional competition that GM did in the States a few years ago: "What can you Hemi" - look it up.

How Fast do You Shake Yours? A Thought Experiment 2 weeks 3 days ago #53046

for my paints I do #4 on your list: "Shake can vigorously for one minute before spraying and occasionally while in use"
for other things a medium stroke is usually good enuf

How Fast do You Shake Yours? A Thought Experiment 2 weeks 21 hours ago #53078

Nice post Jonny,

I usually tend to shake the cans vigorously but i can't tell you for how long i do it.... :silly:

The thing i really take good care of, is that the can and the part to be painted is at the same temperature. I've made "strange" experiences when the paint is too cold or vice versa.
Really nice post !
Proud owner of the Bruiser Family

How Fast do You Shake Yours? A Thought Experiment 2 weeks 5 hours ago #53086

Manotas wrote: ...I usually tend to shake the cans vigorously but i can't tell you for how long i do it.... :silly: ...


Hopefully not for the best part of four years ;)

How Fast do You Shake Yours? A Thought Experiment 5 days 6 hours ago #53264

Slightly off topic but related, my pal who owns a body shop said to improve my rattle can spaying was to have the item to be sprayed at least 20 degree Celsius plus and the can at 40 degree plus, his trick which I now use all the time is to stand the can in a bowl of water at 50/60 degree for five mins before use, then a quick shake(usually a couple of mins). I have found using this method the spray comes out in a better pattern(less lumpy as such) and also a finer spray, once on the item it appears to flows better and has a better gloss to it(on ABS bodies) and also when doing PC shells seems to cover better quicker :y:
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