tamiya primer?

Tamiya doesn't have an acrylic primer.

However, in addition to their spray primer, they do have a brushable, and if suitably thinned, airbrushable primer product, "Liquid Surface Primer" (often referred to on this forum as "LSP". This comes in 40ml jars grey or white and is lacquer based. This product is similar in many respects to Gunze's "Mr Surfacer". Unliike Mr Surfacer, Tamiya LSP is soluble with denatured alcohol and this can be used for cleanup . At a pinch, denatured alcoohol can also be used to thin LSP, but it thins better with  their lacquer thinner.

I went through this conundrum a couple of years ago, wanting acrylic all the way & preferably Tamiya. After looking around it looked to me that there wasnt really much / anything available in acryic primer.

After much reading I came to the conclusion that Mr Surfacer was a reasonable option for priming & after using it I have to agree. It is expensive, but gives a super smooth finish & dries quickly - on the downside supply in the UK seems to be patchy & the product supplied in jars for airbrushing seem to get mixed reviews, I have used this for spot filling but not through my airbrush. All my priming has been done by Mr Surfacer in a can & it seems to work well.

Although Mr Surfacer is not acrylic, this has its advantages as it adheres far better to styrene than any acrylic could & the bond between Tamiya acrylic & Mr Surfacer is stronger than Tamiya acrylic to styrene.

The point of primers is to "grab" the plastic giving you a good surface to adhere the paint to. Which is why you don't find acrylic primers, they aren't aggressive enough. However, if you are patient, and willing to give the paint time to cure (2-4 days depended on temperature and humidity) just use one of the flat light grays. Once acrylic paints are fully cured, they are a very tough surface. Make sure that you have washed off any remaining mold release agents.

A properly cleaned surface doesn't even need a primer coat, though often they show minor imperfections that can be cleaned up before investing time and effort into a complex paint job.

And yes, you can paint acrylics onto a fully cured enamel base.