Question: What resistivity of conductive paint to choose to fix my rear defroster window?

I'll start with a little back story:

I have a JDM bronze-glass rear window in my 5G Civic Hatchback that the previous owner clearly took a scraper to, to remove the tint. In the process, he scraped off about 80% of the rear defroster lines. Very frustrating because he lied through his teeth when he sold me the window.

Anyway, water under the bridge now. I've researched solutions and there are plenty of kits available to fix breaks in the lines. About $10 for 1.5ml at any self-respecting auto parts store. Unfortunately, these are geared towards small repairs, not a complete re-do.

So, I've figured out that these kits are basically repackaged conductive paint, retailing for the equivalent of $6700 per litre. Since I'll be needing so much of it, I thought I'd try some other avenues to achieve the same effect but at considerably less cost. I've settled on some silver conductive paint that should do the trick (from here: http://www.semicro.org/silverpaint-18silver15g-1-1.aspx), though there are copper options too.

My problem is this: I'll need something with some resistivity to achieve the heat effect which is the goal of the rear defroster. But, how do I select which product best matches the existing resistivity of the lines? How do I even calculate this?

The best I can do so far is this: Using my multimeter, I've tested the resistance along the lines of an identical rear window and it sits at about 1ohm over a distance of 1 metre. This was measured at an ambient temperature of 5 degrees C. I think I also need to calculate area and/or volume to get a true measurement of resistivity, but A. this is difficult and B. I don't know the precise thickness of the existing lines, nor could I guesstimate whether this is 1mil, 2mil, etc, since I don't have a good sense of how thick a "mil" is.

The 18% silver conductive paint is rated at < 0.4 Ω/cm sq resistivity. No data is provided for the 36% and 50% solutions, but it's possible this is a near-linear relationship.

Any educated guesses on what the resistivity is of the rear window, and which solution of conductive paint would be appropriate? Do you need more information?

 

By resistivity, I assume your talking about resistance, Im no electrical engineer but I am familiar with automotive electrical systems...
Ive never seen/heard of electrical resistance being measured in ohms/cm sq. I dont think theres a relationship between electrical resistance and surface area is there?.

I think just about every type of heater element Ive measured resistance(resistivity) has been very low, like less than a few ohms. I think I even measured 1 ohm on my dryer heater elements a couple months ago. The defroster is basically a heater element right? I would think the paint with the highest % silver content would yield the lowest resistance, resulting in the most efficient rear defroster...
But im no engineer.....yet.

 

I=V/R

 

Interesting thread. I use conductive silver paint in my lab to detect cracking with via a very sensitive "ohmeter". Be aware that after about one month the silver will tarnish. I can look up what we use for resistivity if you want. It's hella expensive for a little bottle though.

 

You may want to consider entirely removing the existing defroster lines and replacing them with something like this:

http://www.frostfighter.com/prt26STK.htm

 

^^^^
Now thats pretty cool! Ive never seen that before.