Yeah, you are misunderstanding the word 'primer', my friend.
What you want is called a 'finish'/'sealer', which is usually a clear varnish
Matt,
Satin or
Gloss acrylic paint.
Matt-varnish has no shine, satin-varnish has a slight shine, and gloss-varnish has lots of shine.
You can mix them together to get different effects or you can also dilute them with water.
I recommend the 'Vallejo Model Air' range for your Matt, Satin and Gloss varnishes. Dont worry, they can be used with a paintbrush too.
Just dont leave it too long before you clean your brush.
You could also use 'Mod Podge', and dilute it with water for less shine. I recommend the 'Yellow Label Matt Mod Podge'(It actually has a shine to it, despite being a clear 'Matt' varnish)
NOTE: Do not shake the bottles, because you will get bubbles and it will ruin your work.
Just roll the bottle for a few seconds in the palm of your hands or on a flat surface.
...in fact, now that I think about it, I doubt that you even NEED to shake or roll varnish paints.
Ebay links:
Sealers -
(058 - Gloss Varnish)
(059 - Matt Varnish)
(060 - Satin Varnish)
https://www.ebay.co.uk/itm/350690640090?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
Alternative Sealer -
https://www.ebay.co.uk/itm/31047475...eName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649