I'm moving from a steel to an aluminium trailer.  I am going to be mounting some hooks/racks, etc. and will need to screw into the aluminum walls.  Can I use self-tapping sheet metal screws, or is there something else I should use instead?  Any special considerations?

Thanks.

You can use self taping sheet metal screws if they are stainless. Do not fasten steel or zinc coated steel (galvanized) screws into aluminum. Doing so will cause a galvanic reaction, that will damage the base metal, and the joint will eventually fail.

The thinness of the aluminum, will prevent a high loading of the fastener you are installing, as the threads of a self taping screw don't provide much of a grip strength. If the brackets will be subject to much shear or tensional loading, it would be better to use stainless bolts, fender washers and lock nuts to attach the brackets.

Gard

Thanks for the reminder on not using galvanized screws, I would have forgotten.

As for bolts vs. screws, what if I'm not drilling through to the other side?  I'm going to be adding a row or two of hooks to the rear tack, and thus wouldn't want to go through to the other side to provide something for one of my beloved, but slightly-less-than-intelligent-geldings to cut themselves upon. As an example, how would I best hang a rack with four headstall brackets? 

I would use adhesive and the appropriate size aluminum pop rivets.

That's one way. The stress on a tack rack is minimal so self taping screws would also work. Bolts would be required for tie loops, heavy accessories etc.

Gard

I am with retento, go with double sided tape and alum rivets.  You can also use a 'camtainer nut' which is a nut with a with a wide flange, rounded and flat like a carriage head bolt for safety on the horse side.  You use a torx bit to tighten it. 

The installation tool is very nice, but the rivet nuts can be set with a little bit of "Hillbilly engineering".

use a bolt long enough to reach the rivet nut threads and still have enough thread length for a nut and washer.  Run the nut toward the  bolt head, then a washer, then thread bolt into the rivet nut.  Insert the assembly into your blind hole, then set the rivet nut by turning the nut off the bolt.  You'll need two wrenches.  When the rivet nut is set, back off the nut, then unscrew the bolt.  You now have a set rivet nut. 

PS... a bit of oil on the bolt threads makes the process a bit easier.

The use of rivnuts for shelf brackets is a perfect application. They are commonly used in commercial aircraft to secure removable panels. They offer a fully threaded grip area similar to a nut, much more so than the material in which they are applied. Because of their design, they work well when used in a shear application. Think sideways loadings. If you consider them to be an improved wall anchor, ( the plastic insert used in drywall and paneling fasteners) you have the correct idea of their application.

However, I would like to offer a word of caution. Greyhorse has provided a link that shows various applications. http://www.mcmaster.com/ . If you look at the photos, the manufactured flange on the face side, is similar to a pop rivet. The back side flange is formed when the barrel of the nut is compressed during the installation. These two flanges, squeeze both surfaces of the sheet metal in which it is fastened, and secure the nut. The surface area between these flanges is measured in thousandths of an inch. The strength of the application is dependant on the thickness and strength of the material in which the nut is installed. How well the surface material is squeezed between the flanges depends on the quality of the installation tool. A mechanical hand tool is adequate, the pneumatic type is the best.

While it is almost impossible for me to break an aircraft quality bolt in a grade 8, it is very easy to partially install a 6-32, 8-32 or 10-32 bolt into a rivnut, and with a pair of pliers, rip the rivnut out of the surface by pulling on it. A 1/4- 28 nut might take an extra tug to remove. This can be done with one hand when the rivnut is installed in metals of less than .080". Many trailers are constructed of metals less than this thickness. Even it the nut were installed over a frame or stud, it still relies on the small surface area of the flange to provide its security.

If the rivnuts are being used as Greyhorse suggested, for tie rings, you have an accident waiting to happen. The rivnut, has very little strength when used in a tension type of loading. This is a stress I described, where it is being pulled from the surface as opposed to across the surface.

A horse can easily exert hundreds of pounds of force onto a lead, with just a jerk of its head. This is more than enough tension, to cause the rivnut application to fail. The fastened ring will rip from the surface, the horse will be loose, and anyone behind the animal can be injured.

A tie ring should be bolted or welded to the trailer's frame structure. It should never be attached with a rivnut.

Gard 

Some stores may carry them. Check and see if there's a "Fastenal" dealer near you. They carry about any type of fastener that is made.   http://www.fastenal.com/

http://www.fastenal.com/web/locations.ex

Lowes and Home Depot both sell stainless sheet metal screws