window defrost grid

[Dave WM]

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 September 12, 2018 by Dave WM

[S30Driver]

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!

[Dave WM]

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.

[Dave WM]

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.

[rcb280z]

Dave, maybe it has a curing time? Harder to damage after it cures.

[Dave WM]

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 September 16, 2018 by Dave WM

[Captain Obvious]

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.

[Dave WM]

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 September 16, 2018 by Dave WM

[Dave WM]

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.

[Captain Obvious]

2 hours ago, Dave WM said:

(practice makes better)

Haha! I like it!

[Dave WM]

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.

[ensys]

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.