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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 22nd May 2022, 9:38 pm   #1
Al (astral highway)
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Default Spray varnish nightmare!

A while back, I built a very stylish inductor for an RF component requiring massive dielectric strength on the windings. It's OT to describe it and might close down the thread before it's even begun, so please don't ask. Some of you will guess.

I'd done this many times before, building up coats and coats of varnish very patiently and allowing them to cure, either in UV light (sun) or by baking at 65 C in the oven.

I had to put the thing away for a few months and didn't want it to get damaged, so I wrapped it in a decorator's cloth.

When I got it out again, the material had imprinted into the varnish, which had clearly softened. Little bits of lint were embedded everywhere. My heart sank, as I knew that if I operated it like this, these points would become breakout points with arcing that would destroy the whole thing.

The damage wasn't all the way through, but through the last few layers, which I'd sprayed on instead of using my normal brand of spar varnish, which is excellent and remained perfectly hard and cured underneath, so it wasn't dielectric stress as I thought first of all.

I wonder that the spray varnish brand had some kind of additive/solvent to make it cure faster? Anyway, never using that again!

Either way, it took me literally two whole days to carefully remove the mush of semi cured, semi-soft varnish down to the hardened layers underneath. Toothbrush, T-cut (the OG bottle).

Anyone any ideas or similar experiences?
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Old 23rd May 2022, 9:29 am   #2
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Default Re: Spray varnish nightmare!

Hi Al, my first thought is interaction between the spar varnish and the spray varnish...if the former is oil based and the latter water based?? (yes, water based spray varnish does exist, unfortunately!) The fact that the underlying varnish appears unaffected doesn't necessarily exclude this possibility..

When i mixed water based paint with araldite, the resulting emulsion behaved very oddly.

Dave
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Old 23rd May 2022, 10:54 am   #3
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Default Re: Spray varnish nightmare!

Hmm Al,
I am a very amateur painter in all varieties.
I discoverd about 500 years ago when painting motorcycles, that you DONT EVER cover anything with contact materials. By contact I mean anything that can even touch the painted surface, should it even brush by when dry.

I look for a "fridge " box made of cardboard ( refridgerator packing case ) that I clean with a "tack rag ", and stand that very carefully over what I want painted, BUT elevated so a small air flow can cover the drying surface. I dont even dare look at it for at least three days.

Tack rags are cheap and plentiful at any panel beater supply shop. They are basically a high quality cheese cloth impregnated with some kind of "wax " for want of a better word. In any case, they will pick up every tiny hair, dust speck, dead skin cell ( ) or even cardboard slivers left over from the packaging factory.

If you want the ultimate, I used cut a hole in one end, to fit an 8" computer fan. I affix the fan in the normal way. I add a cover over the inlet to the fan itself made from
"scotch brite " ( yep same as in your kitchen ) purchased from the same panel beater supply shop. Another hole cut in the opposite end allows the fumes, paint, and air to escape your " paint shed " .That stops paint droplets heading out to cover everything in your workshop and allows the paint saturated air to escape from the other end of the tube.

Total cost?, two hours work, about $10 ( 5 quid for you ) and you have a vacuum extracted paint room. Its totally reusable many many times. A fridge box is kinda hard to store in its squre assembled state, so carefull dissassembly is easy by removing staples/ glued end flaps, and it can be stored flat. Easily reassembled with a bit of sacricial masking tape.
I hope this helps your Tesla coil.

Joe
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Old 23rd May 2022, 11:23 am   #4
Craig Sawyers
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Default Re: Spray varnish nightmare!

Scotch brite is the standard air filter for old Tektronix scopes that are missing the cooling fan filter.

And tack rags are used in woodworking to remove wood dust before varnishing or French Polishing.

Rather off topic, but it gives a bit of background to Joe's paint booth.

Craig
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Old 23rd May 2022, 11:23 am   #5
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Default Re: Spray varnish nightmare!

Quote:
I had to put the thing away
So how long was this after spraying it?

Also, what actual varnishes did you use?
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Old 23rd May 2022, 12:35 pm   #6
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Default Re: Spray varnish nightmare!

Automotive clear gloss coat in a variety of chemistries... acrylic 1-pack, 2-pack and matching thinners are available from Woodauto on Colne Road. They do liquids in cans and aerosols (not 2-pack, of course!) At least this way you know what you're getting. I've used them as a supplier for decades.

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Old 23rd May 2022, 3:19 pm   #7
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Default Re: Spray varnish nightmare!

One thing I've learned bitterly over the years is not to mix different brands of paint and lacquer.... they often don't like it!

I recall once, having resprayed an equipment-case, I spent ages Letrasetting the various controls/switches/sockets, then to protect the labels I had the idea of misting it over with matt clearcoat, a can of which I hapened to have. At which point the clearcoat attacked the paint resulting in something closer to the black-crackle effect popular on 40s/50s US gear but totally inappropriate in this case.

Different paint-brands. Won't make that mistake again in a hurry.
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Old 23rd May 2022, 3:44 pm   #8
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Default Re: Spray varnish nightmare!

Quote:
Originally Posted by G6Tanuki View Post
One thing I've learned bitterly over the years is not to mix different brands of paint and lacquer.... they often don't like it!

Different paint-brands. Won't make that mistake again in a hurry.

Well, I certainly did break that rule! Good to know I'm not the only one.


David (RadioWrangler) good to hear a trusty recommendation. Thanks!


I'll arm myself with that once I've finished the clean-up.





GMB: ah, I don't remember the spray version now, but in the past I've used Ronseal trade and Sadolin products for the spar varnish. I built up 10 layers of the polyeurathane varnish, left it a month or two and then added another 5 layers of the spray varnish. However, with these I didn't allow much curing time as the varnish appeared to go off very quickly.




Also, I'm wondering. I did finish the last layer of polyeurethane varnish with 2000 grit glasspaper... which leaves a finish as smooth as a mirror. Is it possible that there was nothing much for the spray varnish to stick to?



JoeB. That's a really interesting observation and detailed insight on not allowing contact materials to touch the paintwork. I'll digest that and follow suit.



The Philpott: Sounds like the likely cause. I should have checked more carefully.



Interesting to hear all these experiences and recommendations. Thanks guys.
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Old 25th May 2022, 10:53 am   #9
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Default Re: Spray varnish nightmare!

Decent ozone depleting varnishes are difficult to get these days and are completely absent from the DIY stores.
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Old 25th May 2022, 5:40 pm   #10
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Default Re: Spray varnish nightmare!

Quote:
Originally Posted by PJL View Post
Decent ozone depleting varnishes are are completely absent from the DIY stores.
I like that way of putting it.

What's your top pick?
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Old 25th May 2022, 6:46 pm   #11
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Default Re: Spray varnish nightmare!

I’ve just used some brushing black car paint that I bought back in 1966. It hasn’t skinned over, flows smoothly, covers well and contains all those solvents that are missing from modern paint. Most of today’s paint/varnish is no more than glorified water based emulsion.
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Old 25th May 2022, 7:02 pm   #12
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Default Re: Spray varnish nightmare!

If you want *good* primers/paints/clearcoat, forget the big DiY 'sheds' and instead look at what is still available to the aviation community. Chat up the people at your local small airport and find out where they get their materials. They will have sources for stuff like etch-primer for aluminium, decent CAA-approved paints etc.

Marine chandlers/boating/yachting marinas are also worth a visit; they will know where to get 'proper' primers/topcoats for GRP and plastic parts.

Good stuff is out there, though you have to seek it out!

Pondering the original-poster's probloem [and guessing at what he is using the varnish for] I wonder if one of the industrial high-voltage 'conformal coatings' might be better than using varnish?

https://uk.farnell.com/c/chemicals-a...lating-varnish
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Old 26th May 2022, 1:29 pm   #13
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Default Re: Spray varnish nightmare!

G6 Tanuki,

Good ideas there, creative and suited to my get-it-done one way or another approach.

In terms of the linked product, this family of solvents is familiar to me. For example, I made a bespoke 8kV rated RF cap in an Altoids tin lined with Kalyan tape and exterior coated with such a coating. They absolutely honk and mandatory is a proper mask as they are horrifically damaging to our respiratory systems. They go off very quickly and this can cause a messy appearance.

For large-scale applications like the OP, not so good as they don’t get on so well with complex surfaces that are not completely uniform/ smooth. If I were starting from scratch, also, I’d be able to see if any interaction with the varnish that’s already there. But they definitely have their uses. I also used some as a corona damper on a small area , but still needed 10 coats for that.

I’d be interested to hear others’ experiences of this or more brand recommends for varnish.
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