conformal coating, cheap cutting corners

[GigaJoe]

as usual , my intension as cheap as possible ...

im looking at "Varathane Diamond Wood Finish "  -  polyurethane water base for wood
any opinion to use as  a hobby conformal coating

or something better from auto industry,  like  a top clear coat ?

[thm_w]

Nail polish, clear acrylic spray paint, neutral cure RTV, epoxy and more can be used with certain caveats. One disadvantage being lack of UV tracer for inspection.
If you are looking for as cheap as possible, the spray paint would likely be it.

Unsure on the varnish.

If you want to pay the cost of shipping I can send you some Urethane conformal coat (1L), PM for info.

https://www.eevblog.com/forum/manufacture/selective-pcb-conformal-coat-options/
https://www.eevblog.com/forum/manufacture/best-conformal-coating-for-this-project/

[jpanhalt]

Try Krylon crystal clear.  I would be a little leery of a water-based polyurethane

[jonpaul]

none mentioned are suitable for electronics, expect corrosion, infiltrated electrolytic caps, corroded connectors.

The water and humidity protection is nil on these hobbyists sprays and lacquers.

A true electronic confirmation coating is generally oil based and somewhat thick.

It can be penetrated by probes and peeled off with a solvent.

Jon

[thm_w]

none mentioned are suitable for electronics, expect corrosion, infiltrated electrolytic caps, corroded connectors.

The water and humidity protection is nil on these hobbyists sprays and lacquers.

Connectors are not protected using conformal coating, unless you want to permanently pot them, but I'm sure you know that.

All of these option listed above are fine for hobby uses, and do provide water and humidity protection to various degrees.
For industrial use, no you generally would not use them and stick with commercially rated products. But that is not what the OP is asking about.

[ampedup_electronics]

If I remember right MG Chemicals makes a spray on conformal coating. If it is not applied correctly, it will do more damage than good.

[jpanhalt]

none mentioned are suitable for electronics, expect corrosion, infiltrated electrolytic caps, corroded connectors.
Jon

References? 

I disagree.  Methyl methacrylate and similar methacrylates are widely used in conformal coatings for electronics.  They are clear, can usually be soldered through for repair, and are easily removed with solvents when necessary.  Krylon Crystal clear acrylic spray is virtually identical to MG Chemicals acrylic conformal coating except for some differences in the solvents, which evaporate, and maybe an additional methacrylate   Why pay more than 3x as much for something labeled "electronic."

Krylon:
https://www.conservation-wiki.com/wiki/Varnishes_and_Surface_Coatings:_Polymeric_Varnishes
(Review of Krylon and similar finishes.  The Krylon SDS does not say what resin(s} is/are in it.)

MG Chemicals
https://www.mgchemicals.com/downloads/msds/01%20English%20UK%20SDS/sds-419d-a%20en%20uk.pdf

Sure there are others, but cured epoxies and polyurethanes are not soluble in ordinary solvents.  Of course, there may also be differences in level of protection, e.g., from salt spray.  But, will the TS be subjecting his circuits to salt spray, or is hs purpose just something to protect a board in ordinary household conditions?

[Tomorokoshi]

Similar to those mentioned I use Rust-Oleum "Crystal Clear Enamel" in spray cans. I've gone through probably a dozen cans at least over the years for a certain project. Each board used to get 2 wet coats, but now I put on 2 wet coats, let them dry, and put on another 2 coats. Both sides. Old boards that came back don't show any problems, and they are in relatively harsh service: high temperature, low temperature, high humidity, dust, etc.

I clean all the boards in water with a little Formula 409. Dry thoroughly. Cover all connectors. I use empty connectors with tape over any wire entries. Other things I cover with caps of various kinds. The boards are elevated using stand-offs. It's similar enough to Humiseal 1B73 for my needs.

Before I did this I coated several test boards. No corrosion issues, etc. The only damage is that some of the inspection markings dissolve slightly in the solvent.

Give it about 24 hours to cure. Spray it outside.

[GigaJoe]

so ...
year and half a few boars, outdoor, no any box or cover, under sun , snow and rain
and no any problems at all ...

I use automotive acrylic clear coat ,  that borrow from car shop friend ,  it spray and bit thin, 
apply 3 layers with 2 hours between, then another 3 layers after 48 hours.  it was during +30 summer outdoor ; it dry fast  The layer became kind of thick , and very hard to penetrate for repair. 
everything glued together.
as it automotive it very good for sun UV resistance, no cracking, no even change in color same crystal clear. 
so it very good as protection,  and for repair .... could be nightmare .. mostly due to 6 layers that i deliberately made