Aluminium Metal Powder

Metal powders, contrary to expectations are poor conductors. Firstly, in a metal powder a significant proportion of the material is air. You have a lot of particles which you would expect to be touching each other, which they are, but the point of contact is possibly only a fraction of the surface of each particle (whether it’s spherical or irregular) so you have lots of contact points, but they are quite small, and what you actually have is a lot of gaps, so you have only a fraction of the metallic “continuity” (I couldn’t think of a better word) through the sample that you would expect. Secondly with all metal powders there is a tiny layer of oxide on the surface of each particle, as you would have on the surface of a solid metal, this will give a small amount of resistance on say a solid piece of metal, but multiply that by the thousands/millions of particles in the sample and this resistance becomes significant, building to the point where the metal apparently doesn’t conduct. In reality it does, but not much. There is usually enough conductivity for, say, antistatic properties, but that’s about it. I would think that in a resin, you probably lower the conductivity even more as you will start blocking some of the already tiny contact points. There are metal powders produced for conductivity applications, such as silver coated coppers or even silver or gold may be used (higher conductivity and less oxidation), but as you can imagine they become very expensive.

These metal powders are more meant for cold casting to get a solid metallic effect in a resin. They will not bring out a metallic flake or pearl type effect that you would typically see in paints and some casting resins. For that kind of effect you need specialist metallic flakes and pearlescent powders. PearlEx and other brands offer a range of metallic powders to create the kind of effect you are looking for.

Yes, it would be possible to spray a resin that has metal powder mixed into it however this doesn't necessarily mean that the end result would be exactly what you have in mind. For example, epoxy resins generally don't spray particularly well and when they are sprayed it can often aerate them in an unwelcome way, especially higher viscosity varieties.

In terms of equipment, if you're spraying resin (assuming we're talking about a relatively high viscosity resin such as epoxy) then you would a gun with a large nozzle like a gelcoat spray gun, this would also help to handle the metal powder particles. Gelcoat spray guns like the one linked to can actually be used to spray particles up to the size of sand (for small scale sandblasting) so would certainly be the best bet.

One final thing to consider would be the health-and-safety side of spraying resins. Epoxies in particular would be a real concern when spraying them because they become atomised in a way that they never otherwise would if they were used normally so please be very cautious in this regard and read all appropriate safety information.

...I'd like to use this mix as a means to protect electronics from the elements and as a uniform connecting layer between heat sink and components.

Yes, and no. Adding metal powder to a resin does increase the thermal conductivity and specific heat capacity of the material but the increase in conductivity is less than you might expect because of the way the resin matrix (which is a relatively poor conductor of heat) surrounds each metal particle, to some extent insulating the metal particles from each other. Nonetheless, aluminium powder will increase the conductivity and thermal stability of any gelcoat or resin system it's added to.