Primer depends on what you want to put on it. Some fuel proof model paints will eat their way through Rust-oleum sanding primer. Auto sanding primer will hold up well under nitrates or butyrates. If you are just going to cover with Monokote balsa rite will be fine. I've used carpenter's wood dough and covered with Monokote. For painting I've found that Krylon primer works if you begin, as you should in any case, with a light fog coat for painting. To cover with film just do it. Most hardware stores have the stuff. It has a white lid that just says primer on it.