Call me crazy but..

Interesting,the whole model is all one color? I can't think of anything that wouldn't need more then the color of just  the plastic.

Yes varnish is just like paint,so that's okay and it will give the decals something to stick to.

Unless it's specifically a UV resistant clear coat, that varnish won't do anything to prevent the plastic from whatever discoloration or fading would ordinarily occur. Keeping it away from direct sunlight will be a better bet.

As Tojo indicated, it will help with the decals, however.

Hi;

    If I could tell you the times I regreted clear-coating a car model I built for a review. This little silver Jewel was the Triumph  T-R-7 done by Monogram. Surprise! Not a swirl or Blemish anywhere. When I clear-Coated it it was silver. Now it's Yellowish crappy, Grungy dull Metallic Rust? Color Very faint tinge of silver anywhere. I will Never Clear-Coat a natural color model again-IF it's perfect to start with. 

Sometimes it makes sense.  Tamiya  bikes usually have a couple black sprues that are a dead on match for their semigloss black.

With this kit, I polished the plastic and clear coated.  

I have done that a few times.  However, I always at least clearcoat it.  Styrene is not very opaque- that tends to muddy details a bit.  A clearcoat tends to reduce this effect.

Ya, most model sprues are in their basic appropriate colors, some are spot on.

Example: Tank body body parts are drab/olive green; Race Car struts are semi-gloss black; racing white body; grey engine parts etc.  

So some parts will need painting, others not. IMO.

But it occured to me - all too late - that varnish and water-based acrylic clear might be very different animals. As a novice I assumed they were same thing, seeing as they're both clear. Doh. I sprayed varnish on bare styrene and it looked disgusting so I tried cleaning it off with turps which made it worse so I let it dry then sanded/buffed/shined. Phew, saved. Then I decaled model forgetting to clear coat it first LOL. Or maybe I didn't want to risk same mess happening.

I don't airbrush, and am wondering what is the best type of brush to apply final clear.

The varnish is a spray can, the clear acrylic is not.