I want to paint pine plywood with acrylic automotive lacquer.
What sealer/primer should I coat the pine with before starting with the acrylic auto primer paint?
Or should I just start by spraying with the acrylic auto primer?

What type of finish are you looking for ?

You can spray the pine, but it will look like sprayed pine.

Primer improves adhesion, and will hide some of the wood grain. Auto primer isn't designed for wood and wood primer isn't compatible with acrylic lacquer.

In my opinion, if you want a nice finish, one that justifies the costs of automotive paint, the surface needs to be sanded, and glassed.

After glassing, automotive products can be used.

The model pictured below, is fully sheeted in balsa. I glassed the air frame with 1/2 oz cloth and polyurethane. Automotive primer was used, and the color is from an aerosol.

That makes sense Tom. Thanks.
I would like a smooth, flat surface with little or no grain showing. In other words, I would like to fill the grain.
I do have some 25 gsm glass fabric, which is around 1 oz I think. I have never had anything lighter than that, and I don't even know where I would get some 12 g/sq.mt.
The 25 gsm stuff would do the job, and besides, that is about as light as I want to use for ease of handling etc.
Would not add a significant amount of weight, after all, this is for a 30% scale model at around the 15 kg AUW mark.