You can purchase an inexpensive HVLP sprayer system powered by an air turbine blower for ~$110. This will shoot OSPHO, as well as most urethanes, water based automotive and oil based paints. From Harbor freight you can purchase a gravity fed HVLP spray gun that needs only 30 PSI and a couple of CFM to apply a beautiful finish. This gun is regularly priced at ~$14 and is often on sale for ~$11. I have a friend who did a complete turnstile automotive restoration using this gun. It provides a beautiful finish.

I have several stand alone turbine systems, some inexpensive, one not so. I've used them commercially for a couple of decades, after switching from the older HP guns. For the last couple of years I've switched to the gravity feed HVLP, internal mix guns that are compressor fed. Most automotive shops use this type of equipment. You will pay much more for the name brands, but unless you're a professional painter, you'll notice little difference in the finished product.

The Harbor freight gun will shoot any type of finish other than un thinned latex. If you decide to purchase the gun, also purchase the wire stand that is made for this type of sprayer. Harbor freight has it for less than $10, EBay sales are often double that and add postage. It will hold the gun upright as well as a filter above the cup while it is being loaded. It will also allow you to set the gun down without tipping during or after your shoot.

This is the air compressor I have: http://www.harborfreight.com/air-tools/air-compressors/13-horsepower-3-gal-100-psi-oilless-air-compressor-69269.html  That's not going to be enough to do the job, is it?

Also, now that I'm looking into this a bit further, will I be able to spray upside down(-ish) with any of these cheap HPLV guns?  I need to spray/paint the inside of the horse trailer roof, so I won't exactly be spraying "level."

Can you tell, I've never done this before.  :)

I have the same compressor in a pancake version. It works especially well for my nail guns, airing tires etc. It's light, portable and can be used on any standard household outlet. With a three gallon tank it doesn't have enough capacity for spraying materials or whilst using rotating air tools. If you want to do any spraying, even with a HVLP unit, you will need a larger capacity unit such as this:

http://www.harborfreight.com/2-horsepower-8-gal-125-psi-portable-air-compressor-68740.html.

It has a higher capacity and faster recovery rate that you will need for spraying. Similar units are available at big box stores.

There are a couple of alternate choices you can make. One is adding an extra tank to your "system". This will increase the capacity but will not change the recovery rate. You will be a able to spray for a longer time when using the tank, but then you will have to wait longer to again build enough pressure to resume spraying. This will allow you to increase your system's capacity at a lower cost than purchasing another compressor. Because you will only be using ~25 -30 PSI, the compressor will be able to keep up easier, than when you are using its maximum capacity of 80 -100 PSI.

http://www.harborfreight.com/5-gallon-portable-air-tank-65594.html

The second alternative is to use an air turbine powered unit such as this:

http://www.harborfreight.com/high-volume-low-pressure-spray-gun-kit-44677.html This is often available for $89, and similar Wagner units can be found at big box stores for ~$125

If you decide to use the gravity feed gun, it is often on sale for $11- $12: It performs remarkably well.

http://www.harborfreight.com/air-tools/paint/20-oz-8-CFM-gravity-feed-spray-gun-67181.html

You will not be able to paint upside down. To do that, you would need an HP airless system that costs many hundreds of dollars. With the various HVLP systems, you will be able to turn a gravity feed or siphon gun on its side, which is all you will need to effect almost any project. You can paint at a ground level without your body occupying the same space. Jacking low items can make access to some areas much easier to see.

Hm, that turbine-powered compressor might be just the thing... for the cost of it(particularly because it includes the sprayer, wich I'd need to get anyway), I think I'd be better off with that than with renting a bigger compressor, plus I wouldn't need to hurry...

Thank you for the enormously helpful info!