If you are talking about Tamiya's Fine Surface Primer in the rattle can, I spray all my models with it before painting. I build 1/48 scale exclusively though, so maybe the size makes a difference? Anyway, it really does take a light finger on the trigger or it can build up very quickly. Never let the can stop moving while you're spraying just like when you are airbrushing. I find the primer to be an excellent base for any colour, even white and yellow, and it seems to hide scratches as well.
Anyway, I'm no expert, but I think it's (and others like it) are an excellent product when used carefully. Just keep your subject a little farther away and keep the can moving. It is a somewhat toxic product as well, so spray in an open area (I usually go outside to spray it).
Rich ![Cool [8D]](/emoticons/icon_smile_cool.gif)