Rough and Smooth Textures

I have an antique sewing table that I wanted to repaint using Rust-Oleum's Satin spray paint.  I prepped the piece by sanding it and giving it a good cleaning.  I did several light coats, and let them dry between coats (it was extremely hot outside).  About half of the table has a nice smooth finish, but then there are sections that have an almost grainy finish.  I know I have to start over with sanding, etc, but I don't want to make the same mistake, whatever that is.  What could have happened?  What should I do next?

Thanks!