I ahve done both, mix titanium dioxide powder (TiO2) in epoxy and use Tremclad Rust paint on top of it all. The titanium dioxide mixed well as long as it wasn't clumped up. It does not give nearly the white finish that I got from Tremclad Rust paint. The Tremclad went on well and adhered as good as layering epoxy on epoxy. The Tremclad gave me a brilliant white finish that has endured over the last three years. Consider how bright you want the white to be then decide how you will apply it. Mixing TiO2 in epoxy causes the color to appear a bit yellow because of the natural color of the epoxy and if yo varnish over top of it teh white will be severely yellowed due to the natural color of the varnish. Tremclad already provides good UV protection so you don't have to varnish over top of it. I'd go with the Tremclad. Hope this helps.

Robert N Pruden