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Dave WM

window defrost grid

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I have about 2k ohm resistance and no heat being generated (a few ma at best flow). Don't like the idea of a grid replacement kit that sticks on, and the minute amounts of conductive repair kit only allow for a very limited fix. With my VOM is seems like most of the grids are simply non conductive even though they seem intact.

I was thinking of using an air brush and masking tape to mask off and reapply some kind of conductive paint. see link for a decent amount of paint to work with.

https://www.amazon.com/Caswell-Copper-Conductive-Paint-4oz/dp/B00A6FO6G8/ref=sr_1_1?ie=UTF8&qid=1536694226&sr=8-1&keywords=caswell+copper+conductive+paint

I was hoping that copper head gasket seal may have had real copper in it but from what I have read it is color only, non conductive.

 

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oh and I have some spray graphite I used for dag on old CRT's I may try and see how fine a line I can make on some plate glass just to see how it would work (spray on covering with masked off lines).

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Good luck...hope you figure something out.  I'm the same boat with my car and the rear defroster is the only non-functioning part on my '72.

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I ended up using a fan wired in to the defrost circuit.  It's a 20 amp circuit.  A cheap Oreilly Auto fan got me by the first few years until I got all of my leaks fixed and things finally dried out completely.

Edited by Zed Head

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some good looking products on those links. I just tried it out on a piece of plate glass, approx. the width of the hatch with some of my "slip plate graphite coating". I am sure its not a good choice but since I had it on hand figured why not. If nothing else it will be one of the many products to test. should be interesting to see the resistance reading. Left a nice black line. The masking was pretty easy, with the grid as a guide I suspect it will be very easy. have to let it dry, so maybe a few hours from now I can give a report back. should be able to compute the amount of resistance of a single line, the multiply by number of lines to come up with a  resistance reading to do some current/wattage read out. have no idea of what it should be but figure 10-20 watt would be good, so lets make it easy 12 watts would be 1 amp at 12v. guessing they are about 1" apart and say about 2 feet so about 25 grid lines. I=E/R so 1amp=12v/12ohms, 12*25=300 ohms per line (300ohm in P would be 12ohms total).

so as a target assuming I have the right number of lines, I am shooting for 300-150 ohms per line. that would give me 12-24 total watts. Of course I have no idea if the required wattage is right. Suppose I could go measure the current thru another car with a working defrost, just to see if I am in the ball park

Think I did that right.

 

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big fail #1 about 2 meg over about 2 feet, clearly not conductive enough. Guess I should have figured that, don't think graphite is a very good conductor, couple that with thin line an very thin deposit...

I will look into those other products. I was encouraged by how nice a line I was able to get. I googled to see if anyone had tried to reproduce an entire grid, but could only find the small fixes for breaks OR the complete grid that is solid looking copper strips that have to connected in series to prevent too much current.

Seems like the best way IF a good conductor can be found, would be to have a stencil made up, position it and then shoot the whole thing in one pass.

Edited by Dave WM
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4 minutes ago, Dave WM said:

big fail #1 about 2 meg over about 2 feet, clearly not conductive enough. Guess I should have figured that, don't think graphite is a very good conductor, couple that with thin line an very thin deposit...

I will look into those other products.

Very interesting. It would be nice to find a product that performed well at this. I suspect many older pieces of glass could use some attention like this

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think I will try this one, like the idea of brushing on to build up a thicker layer quicker, and used for track repair (assuming that means PC board tracks) would have to be very conductive.

https://www.etsy.com/listing/519528043/silver-conductive-paint-scp?gpla=1&gao=1&&utm_source=google&utm_medium=cpc&utm_campaign=shopping_us_b-electronics_and_accessories-other&utm_custom1=b66cd0c9-ee2b-42c1-98c1-4d27634cbc4f&gclid=Cj0KCQjwz93cBRCrARIsAEFbWsgTmeVGslyQpamHc6pYAtTZE6mN0NNWoSss2Gc1j9cY4iEpD2s0GgMaAjsIEALw_wcB

 

oops, yikes think its like 15gm, going to have to do some more research.

Edited by Dave WM

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Yeah, I didn't look that close. I did a search for conductive paint to find those. You may just have to look farther back in the search results. Might need to specify aerosol

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I will do some more searching, its not too much and would be good to know if it would work at all. If you can get a really fine line like using a pin stripe brush, prob could keep the waste to a minimum. I think as an experiment I will see how far I can get with a 1/2 oz bottle of model paint just to get an idea of that. I guess this is why they only sell it as a fix for small breaks. too expensive to attempt a whole grid. Anyway will still see how far I can get with the model paint.

Edited by Dave WM

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Watching this with interest.... Would love to fix the only remaining non-functioning accessory on car.

Zup & I discussed this at length a few times,  when he gets back this weekend, I'll get him to jog my memory of the details.

Good luck Dave!

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I went ahead and popped for the caswell copper paint (4oz), 50$ shipped, if that does not work then I don't think I could justify the expense of the other compounds out there just too expensive for a full grid replacement.

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got the caswell stuff in. Seems like its very conductive, I had some issues with bleeding under the masking tape, switch to some green "frog" masking tape which worked much better. Did several tries to see if I could figure out the best technique. Has to be peeled off while wet or the copper trace will just pull up with the tape. Trying different brushes and application thickness to see what works best. will get some pics posted up tomorrow, but so far so good. I don't think it will be very durable, as you can easily scratch off a line. Perhaps it will be a bit more sturdy when painted over the remnants of the old so as to have something to "bite" into.

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hope so, made a few more test lines, I tested it, looks like its about 5 ohms per inch, so will have to go measure the grid and see what that works out to be. Will also  see if that changes after it cures overnite. if that's the case, then 36" wide times 5=180 ohms,/16 grids +11ohms=about 1 amp, not enough to generate any real heat (12-13 watts max).

I will check the resistance again after it cures, and see if adding another layer of paint will get it down to a lower number.  just a guess but I would think 1ohm per inch would get to about 65 watts, at about 5 amps, which I think is prob about 1/2 of what it should be when you consider it used a relay to control the current and not just a switch. I don't recall the fuse size but if it was a 10amp then again guessing 5amp would be a target amount if it had a 20amp then they prob wanted 10amp or about 120watts. to get that I would need about .5 ohm per inch.

One thing for sure, the copper lines sure are purdy!

 

Edited by Dave WM
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I don't have a lot of input that would be valuable, but want to wish you luck with this project. I don't think I ever even tested my defroster, but I assume it doesn't work. My relay clicks. but I assume all or most of my traces have gone open circuit.

Anyway, about the only thing I could add is I have worked quite a bit with membrane keyboards in the past (as a user) and they use lots of silver conductive ink keyboard applications. Silkscreened and cured (either heat or UV cured) after screening. It's durable enough that the keyboards don't smear the material around in the contact area.

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Technique seems to be key, one path is much lower in resistance, sure it has to do with application thickness. Just getting the right technique to apply a thick enough trace that does not tear up when removing the masking seems key. I suspect it will come down to multiply thin coats, PITA but that maybe what it takes. Frog tape is working well. I just laid down a second coat to one of my better lines. My goal is about .5 ohms per inch. that would be about 10 amps, 120 watts. Just a guess but maybe that would be enough. Need to go check that fuse rating. One other thing I noticed, the second coat seems to go down easier over the existing dried coat. With raw glass it can wipe off with the brush leaving a bare spot so have to be extra careful on the 1st pass.

Edited by Dave WM

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update, got it to under .5 ohms per inch with the 3rd coat. Will let it cure overnite for some durability testing. I keep getting better at the application (practice makes better). The plan is to pull the hatch off, leave the glass in, set it upside down and do them all over a weekend. While I have it out I will finally get to install the outer hatch weather strip.

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been a while, got interested in this again after seeing some "stencil tape" so I have it on the way, hopefully will allow a better application of the conductive paint.

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Just a couple thoughts...

I think perhaps there is a good reason that all the old manuals illustrate repairs being done with an old fashion drafting pen (looks like the long bill of a bird). The advantage is that (theoretically) one doesn't have to mask (the bill or "nib" is adjustable for width, and when used properly, will produce a very clean edge. the other positive is that the bead will have a slightly rounded top that would provide more net cross section.

The other is that isn't such a grid usually coated with a thin, clear protective layer of something?

Or not.

 

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The Loctite brand used to offer a product specifically for repairing rear window defrost grids.  Came in a small glass bottle with a little applicator brush built into the cap.

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