# The Fingernail Test



## 804 Paint (Jan 31, 2014)

OK...we talk a lot about products passing or failing the fingernail test. Obviously this isn't a controlled test because everyone's nails and hand strengths are different. 

Honestly, when I do it, it's really the "thumbnail" test...I just gravitate that way I suppose because my thumb is the strongest digit. But anyway, I have very rarely come across a product I couldn't scratch off with my thumb an hour after application, including the newish SW Extreme Bond. The one thing I couldn't scratch off after an hour was Stix I applied to laminate (Breakthrough, Extreme Bond, BIN, Coverstain all "failed"). But I have had Stix "fail" on poly'd cabinetry.

Just been doing a lot of testing lately and really, there is very little I can't scratch off that has even been allowed to cure for weeks.

So how fair is "the fingernail test" really? Maybe my nails are made of steel or something.

Edit: Oh geez, I feel like I need to come back and add...YES, I clean, YES I sand and YES I tack.


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## SkinnyAdam (Feb 21, 2013)

I don't know anything that won't scratch off after an hour, and I've never been called back for an adhesion issue. Not saying everything I've ever done is perfect, but I feel like an hour is an unreasonable amount of time to judge adhesion. Maybe overnight would be a better indicator? I've definitely noticed a difference in products after 24 hours.


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## DynaPLLC (Oct 25, 2013)

804 Paint said:


> OK...we talk a lot about products passing or failing the fingernail test. Obviously this isn't a controlled test because everyone's nails and hand strengths are different.
> 
> Honestly, when I do it, it's really the "thumbnail" test...I just gravitate that way I suppose because my thumb is the strongest digit. But anyway, I have very rarely come across a product I couldn't scratch off with my thumb an hour after application, including the newish SW Extreme Bond. The one thing I couldn't scratch off after an hour was Stix I applied to laminate (Breakthrough, Extreme Bond, BIN, Coverstain all "failed"). But I have had Stix "fail" on poly'd cabinetry.
> 
> ...



Looks like you're doing it all wrong
You shouldn't try scratch off or lift the finish.
If you drag the flat part of your nail and leaves a mark, then you know it's soft.
But I'd give any finish at least a week on solvents and a month for water and oil then test.
I bet you can't scratch a cured CV with your "steel nails"


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## PACman (Oct 24, 2014)

California Ultraplate after a 4 hour dry period will past that "test". If you want to go a little more high tech you can pretty easily and cheaply do a pencil hardness test in the field. But any paint will have to have more then an hour to cure hard unless it is a catalyzed product. Acrylic architectural paints almost always need 3-4 weeks to reach their maximum hardness range. One hour is not really fair to any of them.

The other thing is that you are actually doing to tests in one, which also isn't a great way to compare two different paint films. That's why separate adhesion and hardness testing is what they do in labs, just to keep the playing field equal. Using the "fingernail" test is ok, but it is a pretty basic test and I wouldn't really rely on it as an overall determining factor of a paints quality.


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## stelzerpaintinginc. (May 9, 2012)

Get you some graphite pencils. At least 5. They'vary in hardness. Start with the softest, draw a 2-3" line on the surface. progress until one of those pencils produces a scratch. Repeat several times, record results. 

Your fingernails can vary in hardness for any number of reasons from day to day.


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## 804 Paint (Jan 31, 2014)

Ha, I know I'm just saying that the fingernail test is mentioned here a LOT. Lots of posts, "couldn't scratch it off after an hour." I'm saying I can only recall one instance where I actually couldn't scratch something off after an hour. And, I can often scratch coatings days and weeks after application. 

Doesn't seem very scientific to me, is the point. So I dunno how much credence I put in it.


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## DanielMDollaPainting (Feb 24, 2011)

SW pushes the multi purpose primer to go over oil. My rep says "don't go picking at it in an hour let it dry. I'm embracing it. I've had good success last few times I've used it but I sand everything well. Sometimes even cover stain won't stick good without sanding especially if it goes on a little thin in spots where you can see the yellowish paint coming thru or the substrate is real shiny.


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## DanielMDollaPainting (Feb 24, 2011)

I guess thd general consensus is if it passes the fingernail test it'll hold up against basic wear and tear or at least be good enough till you get paid. Lol


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## PACman (Oct 24, 2014)

804 Paint said:


> Ha, I know I'm just saying that the fingernail test is mentioned here a LOT. Lots of posts, "couldn't scratch it off after an hour." I'm saying I can only recall one instance where I actually couldn't scratch something off after an hour. And, I can often scratch coatings days and weeks after application.
> 
> Doesn't seem very scientific to me, is the point. So I dunno how much credence I put in it.


Not much.


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## PACman (Oct 24, 2014)

DanielMDollaPainting said:


> I guess thd general consensus is if it passes the fingernail test it'll hold up against basic wear and tear or at least be good enough till you get paid. Lol


That's the important thing!


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## loaded brush (Dec 27, 2007)

I have tried just about every bonding primer out there. They will all fail the scratch test within a 24 hour period if enough pressure is applied. You cannot achieve its full bonding effect until it fully cures. Everyone always gives paint cure time to perform its best, but seem to neglect the same applies to primers in order to achieve its full potential. After final finish coat is applied and the primer and topcoat meld together with proper cure time, results should be what you expect from a "bonding primer".


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## PACman (Oct 24, 2014)

loaded brush said:


> I have tried just about every bonding primer out there. They will all fail the scratch test within a 24 hour period if enough pressure is applied. You cannot achieve its full bonding effect until it fully cures. Everyone always gives paint cure time to perform its best, but seem to neglect the same applies to primers in order to achieve its full potential. After final finish coat is applied and the primer and topcoat meld together with proper cure time, results should be what you expect from a "bonding primer".


you haven't tried California's Gripcoat primer yet.


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## PaintMaker (Apr 1, 2016)

Seven days.

Typically as far as manufacturers and their lab technicians go you need to give a coating seven days to dry/cure before testing for adhesion.


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## Epoxy Pro (Oct 7, 2012)

One of our current jobs Stix failed badly. No fingernail needed, just looking at the damn wood work the paint is coming off.


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## PaintMaker (Apr 1, 2016)

cdpainting said:


> One of our current jobs Stix failed badly. No fingernail needed, just looking at the damn wood work the paint is coming off.


Was it raw timber or had it been previously coated?


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## Crackshot (Dec 29, 2015)

804 Paint said:


> Edit: Oh geez, I feel like I need to come back and add...YES, I clean, YES I sand and YES I tack.


trimmed your quote,

I bet if you put your favourite waterbased enamel on. SUPER THIN i mean brushed right out with no bristle strokes (even add 10% of floetrol) bet u cant scratch that off. 

I was like you could scratch it all off. always use that as a reason to show how "****" a paint is... anyway. try that. thin application 
I would brush a bit of undercoated/sanded trim, press it out with a dry microfibre mini roller.

floetrol makes it super tough compared to without. ive tried it on almost all waterbased enamels here in NZ, it also makes it sand to dust also instead of balling up like most do.. which is also a bonus.


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