Personally I don't like putting on the first coat of epoxy with a roller for several reasons. Rollers tend to introduce much more air into the mix than just spreading with a spreader. I also find it harder to get a good amount on to saturate. Having said that, some epoxies are much more viscous than the West that I use and may work better with rollers.

I pour the epoxy on, spread it around at a moderate pace and then pull off excess with the spreader once it begins to set up. It is always better to put a little more on than is needed on the first coat and pull off excess than to not apply enough and have dry spots.

On subsequent coats (fill coats), I pour less on, spread it out with the spreader and then roll it to get an even coat. You will go through many more rollers if that is all you use since they only last so long - fall apart if very wet and epoxy will tend to set up as you proceed on your project - so you will be throwing more away.

Once you roll, you still need to "tip off" with either a dry brush or another dry roller. With patience, it is surprizing how smooth you can get the fill coats with just a spreader and tipping off. I have had customers in my shop that have done such a great job going slow that they only needed to wet sand the finished boat by hand before varnish.