Since this is remotely of related to my line of work, so I'll throw in my 2 cents.
First and foremost, realize that there are 3 ways to heat.
1) Conduction - Something warm physically on your skin like an electric blanket.
2) Convection - Blowing warm air over your skin. Your typical air handler and furnace with a fan uses this principle.
3) Radiant - Heat/energy is absorbed without a medium; just like with the sun. We absorb the heat from the sun, even though there is no medium (like air) that brings it here. It radiates, hence the name.
Typically the preferred method is convection, which is why your typical home furnace uses convection to heat your house. However, most garages are not insulated and as you mentioned, has a very cold concrete slab (it acts as a reverse heat sink; i.e. it absorbs heat energy). In that case, a convection heater is not typically the best choice, mostly because most of the energy you put into the space will be thrown away. To take it to an extreme example, if you were outside to the ambient, blowing hot air towards you would be a very inefficient way of trying to keep you warm. Most of the energy would be wasted and dumped to the atmosphere.
In most poor insulated spaces or spaces with a poor building envelope (warehouses, garages, spaces with roll up doors) the preferred method of heating is radiant. The reason is because radiant heat isn’t affected by convection, winds etc. it is completely dependent on distance. The sun still warms us in the winter even though it may be windy. In fact the reason we have winter is because the tilt of the earth means the sun is at a further distance than in the summer and because the sun heats us radiantly, we don’t receive as much heat energy and thus we have winter.
So in a garage, you should look at radiant sources of heat. The benefit is being that you will still feel the warming effect of radiant heat completely independent of the insulation or how much concrete you have. None of those factors affect radiant heating. However, the catch with radiant heating is that it is completely distance related. Meaning, the further away you are from the source the less heat you get, and the closer you are, the more you get. If you only have a single source for that radiant heat, then you will only have a small sweet spot for an ideal temperature. Radiant heat works best when it is spread out, but short of a commercial system it can be expensive.
If you want to condition your entire garage uniformly then it will be hard to with a radiant system cost effectively. However, it can be done if that is what you want. If you are only going to be working in a small area, like Searchin4acoupe’s set up above, then a radiant heater (which is what he has) is a good option. Radiant heaters come in all sorts of flavors and styles. You can get gas fired ones, electric etc. Keep in mind an open fire and wood burning stove are also radiant heaters although all radiant heaters, by nature, do provide a little convective heating as well. (HINT: Radiant heaters don’t have a fan). I won’t get into all the kinds and brands and recommendations as that will take forever. Just wanted to give you a little background information to make an informed decision.
