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Old 24th Nov 2017, 5:26 pm   #1
astral highway
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Default Varnish on top of oil, for high dielectric strength and looks

Hi folks,

I'm intrigued. I recently put two coats of linseed oil onto a piece of driftwood bark. After a week, I varnished over the oil with marine grade gloss varnish and the finish is deep, lustrous and stunning.

I am going to try the same thing but this time, I'll immerse a pulse transformer in transformer oil (which is a very light grade), let the excess drip out for a few days and then attempt to overcoat with the same varnish.

You might think that wood is a very different medium to wire, and indeed it is.
But what makes me think this possible is that this is litz wire, with a very high capillarity.

Let's see. Could be very useful indeed.
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Old 24th Nov 2017, 7:43 pm   #2
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Default Re: Varnish on top of oil, for high dialectric strength and looks

Linseed oil oxidises and sets after soaking in (giving the lovely finish), most other oils stay "oily" so varnish won't stick.
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Old 24th Nov 2017, 8:08 pm   #3
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Default Re: Varnish on top of oil, for high dialectric strength and looks

Something you need to watch - there's a big difference between straight linseed oil and boiled linseed oil. Straight linseed oil takes a long time to cure unless driers are added. BLO is slower than modern oil finishes, but still much faster than the straight variety.

BLO was the traditional finish for fine gunstocks; an old recipe called for oiling a new stock once a day for a week; then once a week for a month; then once a month for a year, and yearly thereafter to maintain it.

In older military equipment, shellac was to impregnate transformers.

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Old 24th Nov 2017, 8:28 pm   #4
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Default Re: Varnish on top of oil, for high dialectric strength and looks

Danish oil is a good choice too, though the formula varies from one make or another. It's traditionally tended to be a mix of tung oil and polymerized linseed oil, but there isn't a 'standard' formula. Some brands may have resins in them, cheaper brands may have a higher proportion of linseed oil than tung oil as tung oil is dearer, and more expensive Danish oil may not have any linseed oil in it. EG:

http://danish-oil.com/how-we-make-danish-oil/

As MM says, such oils don't simply 'dry' they oxidise and go through a chemical change to form moisture-repellant tough finishes. When used on cabinetry it requires little skill to apply, which is why its favoured by some for finishing wooden radio cabinets.

I think for the task in hand, I'd be more inclined to use oil-based polyurethane varnish as it's thermosetting and won't melt if heated. (many modern varnishes as water-based, and intuitively, that doesn't sound a good idea for encapsulating wound components that will be operating at high Voltages).
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Old 25th Nov 2017, 1:32 am   #5
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Default Re: Varnish on top of oil, for high dialectric strength and looks

If there is no high VOC warning on the tin and it says wash brushes in detergent and water, its no good for electrical insulation.
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Old 25th Nov 2017, 5:13 am   #6
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Default Re: Varnish on top of oil, for high dialectric strength and looks

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Originally Posted by astral highway View Post
I'll immerse a pulse transformer in transformer oil (which is a very light grade), let the excess drip out for a few days and then attempt to overcoat with the same varnish.
One option is to test the oil you are using, linseed or low molecular weight transformer oil (which you can scavenge out of car ignition coils if you don't have any on hand) to see if it mixes with mineral turpentine (I think called white spirits in the UK) and I think both do.

I have diluted linseed oil with mineral turps to repair very dried out speaker cones to give the paper its original quality. Having a lower molecular weight, the turps dries off, leaving the small amount of evenly distributed linseed behind, it can't be too much linseed, just a little is required, probably about 1 part of linseed to 30 to 50 parts of turps.

In any case, for a high voltage transformer you could dilute the transformer oil with turps, this will help it impregnate the windings, with the turps carrying in the oil, then let the turps component evaporate off for a day or two, to leave the oil behind, before you use the marine varnish on it.

(have a look at the Bush TV22 lopty thread, turps is fine on a high voltage winding prior to varnishing it)

Contrary to popular belief, some oils will not stop paint sticking, if there is a thin residue of oil left, because its miscible with the varnish or paint and dissolves into it, that is if it is an oil based paint or varnish. It is just if there is too much oil there the paint will never set and remains tacky.

The main thing to remember is that "like mixes with like" so non polar hydrocarbon chemicals tend to mix with each other, and polar compounds, including water mix with polar compounds.

(One interesting thing, more substances dissolve in water than any other solvent, so never underestimate the cleaning power of a lightly damp cloth which also has a low risk of attacking plastics).
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Old 25th Nov 2017, 12:10 pm   #7
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Default Re: Varnish on top of oil, for high dialectric strength and looks

Hey Hugo,

That sounds like a good method, I can visualise how that works. I’ll give it a whirl with the white spirit base and see what happens. I do have a supply of very light transformer oil here (using it to build your calibrated EHT probe! )

I’ll report back in a few days !
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Old 25th Nov 2017, 3:20 pm   #8
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Default Re: Varnish on top of oil, for high dialectric strength and looks

Quote:
never underestimate the cleaning power of a lightly damp cloth
Or spit, full of lovely enzymes too.
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Old 25th Nov 2017, 5:09 pm   #9
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Default Re: Varnish on top of oil, for high dialectric strength and looks

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Originally Posted by Argus25 View Post
One option is to test the oil you are using, linseed or low molecular weight transformer oil (which you can scavenge out of car ignition coils if you don't have any on hand) to see if it mixes with mineral turpentine (I think called white spirits in the UK) and I think both do.
I've been busy this afternoon, trying wood turpentine rather than mineral white spirits, as a mixture with the very light transformer oil. They mix beautifully and the turps is drawn into the litz wire very well.

I can see a trace of the mineral oil residue left on the ferrite core of the pulse transformer and this is a good thing, as it was troublesome with flashovers.
I think a few more coats and then it looks as though it will take marine varnish very well.


I am also applying this solution to the primary coil of my power resonator, where it will be part of the antiflashover measures I'm applying to the areas of highest voltage gradient at RF. This includes a ring of silicon sleeving which superficially looks waterproof but in fact 'takes' the turps/oil solution rather nicely.

The residue is also very pleasing to look at as it is highly glossy and lustrous.

Quote:
Originally Posted by Argus25 View Post
The main thing to remember is that "like mixes with like" so non polar hydrocarbon chemicals tend to mix with each other, and polar compounds, including water mix with polar compounds.

(One interesting thing, more substances dissolve in water than any other solvent, so never underestimate the cleaning power of a lightly damp cloth which also has a low risk of attacking plastics).
Really good point there about the value of water!
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Old 25th Nov 2017, 5:13 pm   #10
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Default Re: Varnish on top of oil, for high dialectric strength and looks

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Originally Posted by Boater Sam View Post
If there is no high VOC warning on the tin and it says wash brushes in detergent and water, its no good for electrical insulation.

As in acrylic! Yes, I've proven that in the past! Only marine varnish for me now. It's much nicer to work with, for me, anyway. It needs more skill but the finish is much deeper and it's far tougher.

Knock resistance is also needed for my application as the finished inductor will be taken down and stored from time to time.
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