Sounds like your on the right track!
3M,SEM and U-Pol are all good products. Make sure you give the area you intend to put the gravel gard on a good tooth to grab onto... I would not go less of a grit than 320. You may want to put a coat of primer over the gaurd if your using a light top coat(color). I worry about bleeding when light top coats are used... Just me.
If you prime over the gaurd,you can scuff that area with a 3M pad before paint.. a DA or sand paper would cut down the texture. Which would be ok too if you want that effect. (I'm sure you know this)
Just helpfull tips.. sounds like you already have a good hold on what to do as you've already put all the right products down first