Sigman,
Sounds like you were moving too slow and maybe too close to the model. The result was a coat that was too thick and heavy. Remember, we live in a world of gravity. Interesting that no one actually knows what it is, but agree that it exists and can measure it in various ways, even developed units of measurement.
Anyway,
out-smart Gravity by moving the can faster and about 8" - 10" (20.32cm - 25.4cm) away from the model surface. Start spraying before the model and end after the model. You'll get thinner and lighter coats that way. In fact, the first coat should look like a misting and you'll see the plastic.
Practice on a scrap.
For sanding the already painted model: you won't need to sand the whole model. Sand the area affected with the 'sag' and a little outside that area.
First Wait until about 36 hours after you painted it so it will 'cure'. May I suggest wet sanding that area with 'dry/wet' sand paper of 600 grit first then follow with 800 then 1000 or more.
Well, have fun ! (the most important suggestion)