Ian, if you would like a paintable filler, light, strong, adheres well to foam, and is all around repair material for foam you might consider light weight spackling(used to repair holes in plaster and fill nail holes and voids) It is available at hardware places like Walmart, Home Depot, Lowes Home Improvement and places where you live that sells home building materials. Here is a picture of a bit of it costing only $1.25 here in Texas at most any hardware store. It is tenacious in sticking to foam....I have even added whole winglets made of this and they are all doing beautifully. chas PS..it is super super light weight!!![ATTACH=full]54465[/ATTACH] [ATTACH=full]54466[/ATTACH] [ATTACH=full]54467[/ATTACH]
I use the same stuff for skim filling on my models. Over here it's generally known as "One Strike Filler" and it's very cheap for me to get through the trade. Superbly light like you say and can get a perfectly smooth surface with a light sanding. Takes paint incredibly well too.
Whoever said nothing is impossible had obviously never tried slamming a revolving door...