I know this is heresy and will anger some with a more porfessional approach...
But in 35 years of gaming I've never worried about the difference between primer and an undercoat. I've never had a probelm with paint not 'taking'.
Also, Graham Evans introduced me to using common or garden household emulsion paint. He always brushes his undercoat on to his figures using a black emulsion from the local DIY shop.
I've since tried it and can report that no one from the Hobby Police has been around and said that I am doing it wrong. And, to cap it all, the emulsion worked really well. The figures themselves have reported that they feel just as primed for action as they would had I used a 'hobby' specific product.
You can get it in those little tester pots. I bought a 250ml pot from Wilko's for about £5 around 5 years ago. Despite using it for 5 years, you can't tell I've used any. I think it may last me through to my grave. It's actually a very dark grey really, but it works just as well as black. Also, you can get some very nice browns, creams and 'off whites' in emulsion, so whatever your fancy in undercoat/primer is catered for.
Shock horror, I use those really cheap brushes from Hobbycraft or Amazon where you get 10 for £5. to apply it with. I feel like this is some kind of confession and that proper wargamers and serious painters will demounce me as a heretic. But I'm ok with this.