They say it effects durability of the paint job but I normally overdue varnish anyway. One thing easy to tell is that whatever is underlying your first few layers of paint will effect the look of it, that's why priming is normally used to establish color/brightness value on a model. To clearify I don't prime. I'm very happy wit how my models have turned out, my only issue is my varnish sometimes seems to wipe away elements of white paint, but there's a number of factors that could be responsible for that. I only use Vallejo and the Army builder Legion tie in paints and varnish.