I was also a volunteer fireman, working up to Captain in the volunteer company, and then I became the Fire Marshal/ Electrical Safety Engineer for a 3.8 million sq ft manufacturing and research facility for 7 years of my working career.
What kind/size extinguisher did you buy? The little 3" diameter 2 1/2 lb dry chemical extinguishers are good for a frying pan or office waste basket fire, but little else. In most cases, you might be better off not even trying to use it, for safety reasons.
I would get a 15 lb ABC dry chemical extinguisher for your truck/trailer. That's the size that I always have with me. It will put out an engine fire, if you use it right, an electrical or paint and thinner or gasoline fire, as well as a wood and paper fire.
While running between the truck and the fire. tip the dry chemical extinguisher end to end several times to unpack the powder inside, or it won't all come out with the available propellant. It packs well from vibration in a vehicle. Don't plan on using just a little of it, because once the powder is in the valve, the propellant pressure will leak down very quickly and the rest will be useless when you can't get it to come out. Commit the contents of the extinguisher to the fire and be certain that you have put the fire out completely before closing the extinguisher valve. Also, don't shoot it at the flames or smoke. You need to fight the source of the fire. It will work best if you use a low sweeping action toward the base of the flames, to kind-of sweep the flames off of the burning material. You can put out a lot of fire with this size extinguisher if you use it correctly, but be careful not to let the flames circle around to re-light what you have already put out. You have to do this right and once, because you won't have enough extinguisher left to try it again. You also need to be close to the fire before you begin to use the extinguisher. As close as you can get is best. If starting back at the extinguisher rack, it will be empty before you even reach the fire. You only have seconds worth of extinguishing material after you squeeze the trigger. With a 15 lb, maybe 15-20 seconds. Make it count.
There are class labels on extinguishers. Class A is for burning wood/paper, B is for paints, solvents, gasoline, etc. C is for electrical, but when you cut the power off it usually becomes a B fire. D is for burning metal like magnesium. I doubt you will ever need a D type extinguisher. The best for your needs is a Type A, B, C dry chemical extinguisher, because it will work well on most any type of fire. It makes a huge mess, but it will work to put the fire out very well. C or BC type extinguishers are usually carbon dioxide CO2, and they will work well for most flammable liquids and electrical too, but these extinguishers are heavy by comparison. On the plus side, there is no mess to clean up afterward. Unfortunately, they don't do well for paper and wood fires. The 20 lb size is the best to have. I can now put out about a 100 X 100' area of burning fuel oil with just one of these 20 lb CO2 extinguishers, but it takes practice.
Charley
), started gluing the maple blocks in tonight, will finish the rest tomorrow morning since I used all my spring clamps for the first half of them. 
