I would sand it with 400 with an air file, or 600 with a 12 or 15" block, pick one up on ebay or Northern tool. sanding with an air file will leave finer sand marks than by hand sanding, the blotches ethier come from the work that has been done under the base, or not getting good coverage when you spray, I’m not familiar with the atom so I can’t really suggest any settings for you.“Watch Tony’s video on how to adjust your gun” The blotches may also be coming from incorrect gun settings or spray technique. Make sure your not holding the gun to far away and spray 50% overlap with base and 75% with the clear. IMO you should always use a sealer before base, it will help block out the body work and fine sand marks and imperfections. Also the quality of the base coat your using will determine how many coats to spray and how it covers. Be patient and let it flash before applying the next coat…
Also looking at your close up picture it looks like you may have just sprayed a little to fast and didn’t get the coverage or the air pressure may have been a little to high, painting in an uncontrolled environment is a little like baking a cake, the temp., and humidity plays a big part, so what worked one time may not be the same the next,… gun pressure,mixing, type of reducer, technique, etc.
All in all it looks good for your first time.
I think but not positive, Tony recommends the LVLP for clears. I use a completely different gun.