Questions: Filler Putty over primer?

Hey guys, I'm new to this forum and have been reading a lot of the posts here. Good stuff! I haven't looked until recently for techniques or ideas and stumbled across this place. I think I'll stay if there's room. :)

I've been modelling off and on for the past 15 years. Never enough to be "really good" at it but I try. I recently just started building up a collection of Tamiya motorcycle models as well as Tamiya acrylics. I dug out my old Testors Model Master airbrush (basically the same thing as the Aztek I believe) with a CO2 tank. Still experimenting with all that; basic suggestions about this setup are welcome.

I've got a couple questions (the first of many to come later down the road) regarding primers. For what its worth I'm using the Tamiya grey primer and also picked up a can of the "fine" white primer (which I have yet to try). I've read that a lot of you guys simply use whatever's cheapest at Walmart or the local auto parts store. If it works just as good I'll give that a try when these dinky cans run dry.

My first legitimate question is whether or not it's ok to apply filler putty (I'm using Squadron Green, thinned with nail polish remover - thanks forum!) directly over a primed surface? I just primed a part that required a seam to be filled right down the center and have noticed that there are a couple of spots that need to be filled further. Do I need to sand down the primed area before adding putty?

Next question: I know (well-I read it anyway) that primer influences the tone of the color that is sprayed over it. The model I'm currently building is a Ducati 916 that is red. Will another primer color besides generic grey enhance the red when everything is said and done?

That's all I'll leave you guys with for now. I'm sure I will have many more to come!

Thanks..

Jason