Glyn

Firstly you must be very wary of varnishing over acrylic. If the acrylic was water-based and the varnish is solvent or oil based then it likely won't stick properly and will peel off quickly. Much the same goes for an oil-based top coat and a water-based varnish. To be safe you need to use a clear varnish of the same type/manufacture as the white topcoat.

Next, some varnishes have UV inhibitors which turn the varnish yellow after a short time. These tend to be marketed as external/outdoor or yacht varnishes.

As regards polish and T-Cut, it depends on how much of a perfect finish you want. You can also get really ultra-fine grades of Wet & Dry these days but you certainly need a good depth of paint to be able to rub down to a finish. Do bear in mind that as soon as you apply anything like T-Cut or polish to a painted surface then the chances of getting any further coats of paint to adhere properly are very small.

For what it's worth, I only rub down my models as far as the final primer coat and I never bother with polish or T-Cut on the top coat.

Dave M

Edited By Dave Milbourn on 17/05/2017 10:34:17

Glyn

Car paint will definitely be waterproof – or they wouldn't use it on cars! BTW most paint manufacturers will produce a clear lacquer to put onto their metallic paints. Halfords' aerosol range includes a satin lacquer, too, which is handy for reducing the "toy" appearance of a scale model which has been painted with gloss.
If the present finish will withstand immersion in water then there doesn't seem to be any reason to varnish it – as long as you're happy with it as it is.

DM

Almost any transparent coating over white will give a slight colour cast if only because of light refraction through the transparent layer. If a white finish is needed then make it the actual final colour coat.

White comes in matt, satin or gloss finishes in most paint ranges.

Colin