Problem after sealing MDF

[ellal]

I was recommended to seal MDF with Zinsser 123

However after doing so, small 'filaments' of the MDF came up.

I very lightly sanded with 240 grade, and then applied a water based undercoat, but the 'filaments' came up again.

What am I doing wrong?

 

[Munroast]

you just have to keep going, more paint and more sanding over and over again till you win.

I used some old 'chalk' paint the last time and it seemed to work well. - I don't think the expensive specialist paints for MDF edges are any better

I once used that water based special metals primer (the red stuff) and that worked really well, and drys super quick, so fast that it could be sanded and redone within the hour.

 

[ellal]

Just in case it is of interest to anyone...

I still got the 'filaments' coming up after several, so was recommended 'Flag MDF Primer'.

Toolstation had some old stock - they no longer sell it.

I used it on an identical sheet of MDF and it worked a treat!

 

[opps]

All water based paints cause the surface of MDF to "sell".

I spent many years as a cabinet painter. I used to use Leyland Acrylic primer even though I knew it would raise the grain. I didn't mind though because I was aware that MDF is not as smooth as the manufacturers would have use believe. I used to sand it back with 180 grit silicon carbide (hand sanding) or Festool 220 grit Titan paper on an interface pad (machine sanding) until I could no longer see the parallel sanding marks generated during the production process.

Once sanded I applied oil based eggshell, which doesn't raise the grain again.

BTW, I used the cheapo Leyland Trade acrylic primer/undercoat because it is a dream to sand. Most waterbased primers are a mare to sand. The sanding process pretty much removes 95% of the surface primer but it has the added advantage of allowing the OB eggshell to soak into the MDF and give a uniform finish in terms of opacity. It also meant that I could get a high quality finish with two coats of eggshell. The paints were applied with Anza (free of foam) paint pads. Had I been using a roller the process would not have worked quite so well.