priming mdf

We fitted a new staircase last week. The steps are MDF and frame is timber. I started painting the stair at the weekend and used a diluted coat of emulsion as an undercoat. A second coat of the emulsion was added and today I painted the staircase with gloss. After 6 hours, I used a damp cloth to...

I have several door frames and architraves to paint. I’ve sanded back alot of the old gloss off the frames. There are some knots to treat. Although the MDF is primed I had to sand some bits back so that needs re priming. I have Zinnser BIN, should I spot prime the knots and use some kind of...