Painting Resin - Varnishing before?

[Grim Dog Studios]

I read somewhere that by varnishing forge world models before undercoating helps the paint "stick" to the model better. I'm never confident on whether I've washed the resin from FW thoroughly enough before hand, and I've had cases before where the paint "shrunk" after application, however this could possibly have just been a bad spray can of black undercoat (I didn't dare use the can again).

Any help would be much appreciated, and if varnishing before hand does do the trick will this army painter matt varnish spray do the trick?

http://www.waylandgames.co.uk/army-painter/sprays/matt-varnish-spray/prod_3328.html

 

Thanks for looking!