Fool Proof Finish

Hi All,
I flew Canadian .25 Scale Combat for 20yrs.
Foam/balsa/ply against amazing coro designs. (very foolish)
Somehow,
The fuel proofing I used worked out well.
I bought a can of semi gloss water based polyurethane varnish. $10.  WPV.
This replaced the epoxy/dope I used to use.
The basic design is a 1/8th ply fuse that holds the engine/guts/wing/tail together. Tail is balsa.
The 'shape' of the fighter I want is sanded white styrofoam.
Wing is hot wired white foam, light glass cloth put on with WPV. (instead of $stinky epoxy)
The plywood nose area gets soaked in Dollar Store CA. (key point)

So, everything is together, ready to paint...
With a nice brush, put a light coat of WPV on everything. Goal is to get to the last piece, as the first piece is ready for the next coat. Another light coat. (like dope, but does not stink)
Divots in foam get filled with light spackle. If you get it right, you can burnish it down to where you do not need sanding.
Next day,
It seems soft, that is good. Fix any parts you don't like.
This is the weird part.
It is not fuel proof.
Until,
A coat or two of outdoor latex(acrylic)semi gloss house paint.
More weird, thinned with blue windshield washer fluid. ( I do not know why, I swear !)
If you have brushed on Buterate Dope, you will love this stuff.
You can get any colours you want.
I prefer getting a can of 'Neutral Base'. (dries clear )
And mix and match Dollar Store cheap acrylic paints. 50/50.
The magic is somehow,
The Latex/acrylic sticks like [bleep] to the WPV base. And I have no idea why,
By the time you have done your first light brush coat, you can touch up till you are done. Dries fast.
Thin milky coats, no rollers involved.
Results?
Great.
Unless you like dumping 1/2 gallon of 15% nitro on it,
Then rubbing it with steel wool.
I do not know, how, why it works.
I am guessing,
The latex, gets in the way,
Between the Castor and Balsa....

Touching up is fun. Semigloss rules.
AND ! You clean any oil off with,
Windshield Washer fluid ! (this freaks me out !)

I love all the old ways,
And all the new ways, to figure them out,

Respect to All,
Dave