Shrinkage

[El_Duderino]

When clear and primers are left in mixing cup and watched for weeks as they cure, why do different brands shrink more than others? Why do some split open? Does that mean much for quality? Is that an indicator of when it is cured and is best to begin with the next steps?

 

[mitch_04]

I've been doing a redneck test where I leave a couple ounces and let it dry. I'm going to measure the dried pieces compared to the cup and see which products shrink the most. Those should be the products you would want to limit the build of or let dry the most before you sand.

Just another reason not to rush the steps, no sanding scratches to come back!

 

[shine]

invalid test. no clear or primer will hold up if applied 1/4 in thick.

When clear and primers are left in mixing cup and watched for weeks as they cure, why do different brands shrink more than others? Why do some split open? Does that mean much for quality? Is that an indicator of when it is cured and is best to begin with the next steps?

what happens to a hockey puck in the cup has no bearing on the product .

 

[mitch_04]

I'm not worried about the splitting, but it has to be somewhat valid for determining which products shrink more than others... Right? If not, please explain why.

 

[Jorge M.]

I'm not worried about the splitting, but it has to be somewhat valid for determining which products shrink more than others... Right? If not, please explain why.

The rate of shrinkage in a cup could mislead you to think a product is better/worse than other. SPI primers flash of so fast that at 70 F you can dry sand them in 30-40 mins, this means they will shrink faster in a cup than other slower primers, the difference could be in days. So you can see how some ppl may be fooled into thinking they're of inferior quality. This was explained to me by Barry btw.

I've kept many leftover clear pucks in a cup for many months, the only difference I could tell is the clarity and flexibility of the pucks.

Only two clears I've come across that looked like a dried up piece of oatmeal, maybe those were the only ones really really low in solid contents.

I'm sure there has to be a way to gather info from either the TDS or the actual label on the can.

Aren't there new laws that are forcing paint manufacturers to disclose more info on the labels, some of it related to the % of solid contents?

 

[mitch_04]

I could understand the misleading of quality, but I'm purely concerned with shrinkage. Basically, I just want to know which products I should let dry for extended periods of time before sanding. Also, which ones I should try to limit my use of. As in, if 2k shrinks much more than poly, I'll try to use more poly. If epoxy shrinks the least, then I'd like one of my last coats to be in epoxy with a good cure time to reduce my chances of sanding scratches or filler outline returning.... that sort of thing!

I enjoy conversations about these things, 2 years ago I had no idea that one product shrunk more than another and the problems that could arise from this.

 

[Chris_Hamilton]

Like Shine said shrinkage in a cup and shrinkage on a vehicle after it is applied are 2 different things. Apples and Oranges. Main causes of shrinkage like you are describing are misapplication of the product (improper flash time, applied too heavily etc. etc) and not removing coarse sanding scratches, not the product itself. If you were using lacquer primer you would have to factor in shrinkage but thank God we don't have to use lacquer primer any longer. Anything that is activated/catalyzed will have minimal shrinkage if applied correctly. If you are using SPI follow the procedures outlined in the Tech Manual and the Perfect Paint Job article, if you are using another manufacturers product adhere to the TDS and you shouldn't have to worry about shrinkage. If you are really wanting something perfect, give your final application of primer a week or longer sitting in the sun. Junior Conway used to let something sit for a month before final sanding, but he started doing this when lacquer primer was all that was available, he continued doing this with urethanes probably more out of habit than anything else. Epoxy or urethane a week in the sun will eliminate any detectable shrinkage.