- smartphone: Samsung Galaxy S4 mini
coating is probably crome, a few microns thick
could it be that removing the coating will have the opposite effect, as in lowering the signal strength instead of increasing it? Because the coating is designed to reflect/focus/whatever the radiation in a purposeful way?
- or is all of this irrelevant because the antenna emits/receives radiation only in certain directions anyway?
Telephones are designed as a system. So, yes, the plastic case, including a potential coating, have been incorporated in tests, and in consequent optimization (at least for large-scale mid- to high-end phones). Chances are you will not be improving anything.
The metalized surface will be thinner than the skin depth at relevant frequencies, and quite possible not as conductive as you think (I'd be happy if proven otherwise).
or is all of this irrelevant because the antenna emits/receives radiation only in certain directions anyway?
While technically, all the antennas in the phone do have a certain direction in which they radiate most of the energy, it's not like a phone's antenna system is designed to be highly directive – that would be counter-productive for reliable reception.
I once removed the cover of an HTC One X for cleaning purposes. As curious as I am, I switched it on and noticed really poor reception. Upon further investigation, I concluded that some metal parts remaining on the cover are actually the antenna.
Googling your phone, I found https://wiredornot.files.wordpress.com/2013/08/img_20130807_132347.jpg which shows several put on contacts on the covering plate. If you remove it, you will certainly disable your loudspeaker und I think the audio jack. Unfortunately, I couldn't locate the antenna. It could be on the other side of the board or one of the remaining put on contacts.
The coating could be there to prevent micro-oscillations in the circuit itself. It was definitely factored in to the design of the phone.