Smoothing lines for painting

[Millwright]

After some cable chases were plastered with one coat plaster, I'm having an issue with getting the edges/lines smooth.

I can sand all I like and it appears to feel smooth, but once I paint, the join line is more pronounced. So I'll wait the next day for it to be dry and go to town on it again with sanding, lovely and smooth. Then I paint it and the line is apparent again.

What might I be able to do? What might I be doing wrong?

Thanks.

 

[Pigeon85]

Could you post a picture?

Is it the new plaster sticking out proud?