Easiest way to paint a Harley Benton kit.

Corvus

The nozzles is a very good point, in paint/body shops you get a choice of nozzle. You can buy them online nowadays I think. But whether they fit a particular can is another thing. The flat-fan shape is far better for covering areas than the splat dot shape.
On which point, car paint places make up good paint made up in cans. Bodyshops often get a can made up when they only need a little bit of basecoat for a repair. Whereas Halfords and anyone else branding someone else's paint likely buys it from different sources over time (I would think), more chance of quality variation. They certainly don't make their own.

On the porosity front, solvent-based paint needs the solvent to get gone so it doesn't really matter what it goes down on. But it may have disagreed with something underneath it like a sealer or primer or something. The solvent goes in a matter of minutes (unless laid on thick in which case it can get trapped) and the paint crosslinks. But there's always a f-up factor with paint, weird things can happen any time. e.g. I had some discolour in blotches underneath 2k clear, a month later... no idea why. It's not 100% predictable (annoyingly).

Those turbines like Apollo and others - not sure, some people like them but I've yet to see a really good job laid down with those - but only going by web pics only (cars etc) - they always look orange peely. Which you can sand out I suppose if you're prepared to put up with that.
I come from the car & motorbike side of things and not many people would choose one over comp+gun. If it was me, for guitars, I'd get a small comp second-hand and a small gun. You can choose guns, replace them, use different ones, etc. Change comp later if you get hooked. Flexibility and control and choice etc.

Or stick with cans from good suppliers, especially if it's just the odd one or two you want to colour in.

thermionic

I sprayed a pedal enclosure using some Halfords metallic car paint - Nice colour and it didn't chip but a month later it was still picking up the indentations from my fingerprints. A year has gone by and it's still so soft it sticks to surfaces I leave it on. I have to remember to move it around every now and then!

bobliefeld

I've gone with some bits from Rothko and Frost.  If it turns out looking ok I'll put up some pics.

I put together my "T style" kit last night and surprisingly after just screwing everything in and putting strings on the intonation was pretty much fine and it plays ok. it's a little buzzy but nothing that can't be resolved.