Hi Evan, why not just use a basic spray primer from the hardware store, instead of an expensive imported modelling product?
I've use an automotive primer for most everything for years with superb results, and once I got into the habit of warming the can first I've never had the primer fill detail. I'm sure someone can recommend an equivalent for over there (Rustoleum?) Anything like this is going to work out waaay cheaper and they are really tough - for the record, far better than undercoating in Humbrol, which is pretty fragile.
Mr. Surfacer is intended to fill surface defects as you prime so it won't be ideal for a general-purpose primer (it will tend to fill fine detail) but if you want that feature you can buy sandable fillers from automotive stores too, again they'll be much cheaper than any hobby product if you can find the same there.
Einion