Originally Posted by rbryan
Very good point. I use wet/dry. I'd start with 400 grit but for a really smooth surface maybe finish with 600. Since the OP intends to tip and roll instead of spraying, and the tip and roll leaves a very slight texture, I would think 600 is the finest you need to go.
I would not do 600 if I were you. Paint needs something to bond to and 600 is way to TOO smooth a surface for MOST paint systems to properly adhere. Do what the manufacturer of the paint system YOU CHOOSE recommends for surface preparation.
Remember this is a TRAILER and not a $50K collector car. It's a safe be that your trailer will sit OUTSIDE therefore a quality paint product that is properly applied with great UV resistance is far more important than a "Glass Like" finish that will be color sanded and buffed.
You need to specify the brand,type of paint and method of application you intend to use on your trailer to get proper advice. That will dictate the level of (tooth) or surface preparation you will need for the paint product you are using to properly adhere to your trailer and provide all the benefits the paint system you use can provide.
What Brand of Paint, Solid Color, Metallic Color and type (Epoxy, Enamel, Base/Clear, Single Stage) of paint system will you use?
Will you "Primer" the complete trailer first or just spot "Primer" over repairs as needed?
Will you be applying enamel over lacquer? Could you possibly have a paint reaction between the "New" Paint system and the "Paint" currently on the trailer?
Will you be using a "Highly Loaded" Metallic paint? This will limit your paint applications methods to spraying for the most part.
Are you spraying, rolling or brushing the paint system you choose?
Is this method of application you choose recommended by the Paint system you choose?
These are just a few of the questions you should consider before you decide how to prepare the surface of the trailer prior to painting.
As stated above PICK the paint system you intend to use, follow THEIR instructions, ask questions here as needed by providing us all the information you can and have at it!