mc222

What type of place do I go to , to have my paint density checked?

Prior to receiving my car, the dealer repainted the nose of my car in order to bring it "up to spec" and make it Acura Certified.

Now I notice that places on my front bumber are chipping and the paint seems very thin. So I want to measure out the paint density and see if I can get it warranteed...

Anyone have any clue where I can go? If I go right to the dealer they will probably feed me a line of BS..

Smitty

Go find Dave.. he will have your answer... see below

dfreder370
A-CL Senior Member

Registered: Dec 2002
Location: palatine, il
Posts: 503


Something we should all be aware of, our sheet metal is 0.8 millimeters thick, or 0.032". That is about as thick as six sheets of computer paper.

mattg

you should be able to pick up a magnetic mil thickness gauge at a paint supply store, or on the interweb.

dfreder371

A magnetic thickness guage will not work. Our bumpers are plastic. If you can remove a chip, you could use a micrometer.

But your question was about paint density, or rather the specific gravity, or maybe the viscosity of the paint. There are three ways to do this. One is to measure the viscosity with a zahn cup. The zahn cup has a calibrated hole in the bottom. You immerse the cup in the paint and measure the time it takes for all the paint to run out. A chart is then used and the seconds can be converted into what we use to measure liquid viscosity: The units are called centipoises. You can probably take your paint to any body shop and they will do it for you.

A better way is if you know someone who works in a chem lab, like me. I have a brookfield viscometer, which has a very delicate and calibrated tortion spring. You spin a spindle of a certain size at a certain level, and when the forces equalize on the spindle, you "trap" the needle which will be a number between 1 and 100. Once again, a chart is consulted. If you go this route, the spindle and spindle speed should used to get a reading between 20 and 80 for accuracy. This is a very simple and accurate way to determine paint viscosity for someone who knows how to use the viscometer.

Method #3, probably the easiest would be to get a pounds per gallon cup from a scientific supply house like VWR scientific or Fischer. You put this on a good gram scale-one that can weigh out to 0.01 grams is needed. You fill this to the top with water (what specific gravity is based on) then put the top on. Some of the water MUST escape out the hole in the top cap. Weigh this whole thing. Ditch the water, completely. Now weigh only the cup and cap and subtract the cup and cap weight. Its been a while since I have used one of these, but I think they hold 100 grams of water. Now do the same thing with your paint. For example, if your paint weighs 84 grams, 84/100= s.g. of .84.

For liquid paint, automotive, that is, the viscosity should be right around that of milk. In other words, not very thick at all. It is usually common practice to dilute paint one part paint to one part solvent, by volume.

Your car should have a clear coat thickness of 1.5 to 2 mils and a color coat of 2 to three mils. 1 mil is equal to .001".

If you are REALLY concerned with DFT, or dried film thickness of a non destructive nature, boy, that is a tough one. You are going to have to find someone like BYK Gardner and ask them to measure your paint film thickness. Don't even think about buying one of these instruments. They start at about 2 grand!

And, it is too late to do anything about this now, but here is how they do this with liquid car paint. GM, and Acura can afford to do this, but you can't. They make a little tool, called a wet film thickness tester. After the paint is sprayed and still wet, this little tool that kinda looks like a pocket comb-but the depth between the teeth vary, gradually, in thousandths of an inch. You drag this through the wet paint, and notice how many thousandths get filled in. Lets say it was 8 mils. Now the paint gets cured and a cross section of the paint and whatever it was on gets cross sectioned and looked at under a microscope. There are lenticulars (eyepieces) that can measure the dried film thickness in thousandths of an inch. If the DFT is 4 mils, and the WFT was 8, you now have a relationship established. WFT=12, DFT should be around 6.

If you clarify you question (I think you really want to know the DFT), I can be more specific.

I know Smitty is gonna flame me for being long winded. Don't ever let me get started about paint!

mattg

cliff notes?

btw, he obviously meant thickness, not density.

a thickness gauge will work fine except for the bumpers.

mc222

LOL - I understood a little bit of this and I thank you VERY much!...Basically, I guess I need to find a shop w/ a BYK Gardner machine....Hmmm, Ill make a few calls -

but I do have access to a viscometer believe it or not - My friend just told me he uses one at work all the time ( he works for J&J )

Can I just give him a small paint chip in order to measure out the viscosity? And how will knowing the viscosity help me determine if the paint is "up to standard"?

mc222

Damn - My boy at J&J backed out on me....

"heres the thing, I would but the stuff that we measure is calibrated for liquid medicine that goes to kids. I would be afraid that even the slightest (minute) paint residue would through off future specs, not only that it would contaminate future samples, so I shouldn't play with it. Try benny at my aunts shops he mikes paint and paints cars on a regular basis, he might be able to help you. Sorry,"

dfreder371

I think we need some clarification of terms here.

Specific gravity: Density of a material relative to water. Water specific gravity=1 @ 8.36 pounds per gallon. Gasoline S.G. is <1 at 6 pounds per gallon. Some highly protective (and now illegal to produce paints) have an S.G. of 4, as they are LOADED with lead. This paint is called SUBOX, and is used by electric utilities to protect large and expensive high power transmission lines found here in Illinois. By the way, my father designed these poles. I have also seen them in Pennsylvania.

Viscosity: a measure of fluid THICKNESS ac compared to water. Heinz ketchup, and maple syrup have a VISCOSITY that is higher than water. Gasoline has a lower viscosity than water. And it also follows that 50 weight motor oil is thicker than 40 weight, that is thicker than 30 weight (simplified).

I think what you want is to know your dried film thickness. Correct?

mc222

Correct

Smitty

Hey I referred him to you because your the best!! and I knew you could answer his question.. no flames... I am just glad to see you did not start off the thread with "Hey it's me...Dave, the powder coater"....:P

Smitty

dfreder371

Thanks, Smitty.