Horse trailer with minor surface rust on bottom. How to fix? Cleaning product recommendations?

If it's just surface rust you can wire brush or sand it down to pretty clean, then use a product like POR-15 or Chassis Saver and paint over that later on. You can get away with a decent METAL primer then top coat that also. If you use POR-15 or other similar products you can treat the bare metal first. So, the level of what you want to do is dictated by how much you'll use the trailer and also how long it just sits. Let's face facts, horse trailers can rust pretty quickly because of the physical use AND the wonderful organic waste products that can interact with the metal so using the best product initially is normally the better option long term too. Aluminum has corrosion and steel has rust, with steel you can encapsulate and then cover the rust, convert the rust and cover/seal then paint anything later. I worked on KC-135 aircraft and now working on a rusty 55 F-100, both need different approaches to dealing with bare metal and corrosion or rust.  

On a trailer, just like a car or truck, you need to wash it regularly and make sure the weep holes drain and nothing gets clogged up, more so with a trailer you need plenty of rinsing. 

About 30 years ago we bought a 1973 two-horse Kingston. We ripped out all the floorboards, wirebrushed the frame, and painted with five coats of Rustoleum, starting with three coats of the rusty-metal primer. We still have the trailer and still use it.

Best thing to do is wire wheel the rust off with your drill. I've used the Rustoleum primer that comes in a one quart can to cover metals that I have wire wheeled down to bare metal. It's a rusty reddish in color when dry and is the same stuff they use on Navy ships. Has a high Zinc content. 

There's a facebook page called Horse Trailer Conversions that has saved files on it of all sorts of repairs.

Years ago my friend Cheshire bought a used horse trailer with a similar problem. I don't remember what sealant she used, but I vividly remember her description of renting a sand blaster to remove the rust and what a good job it did.