Is it OK to use a airless sprayer such as the Wagner 117 project pro to spray a newly built home which has taped and seamed joints? I have heard that this can cause blisters to the joints and may not be the best way. Time Is a big issue

I recently painting my bedroom doors with flat enamel, but we now don’t like the finish. We now want to paint them with semi gloss enamel. Can I directly paint over the flat enamel, or do I need to sand or prep the door in some way before I paint