The only product I could find here in the UK was "International MDF Primer".

I used it to prime some MDF loudspeakers I built (with a brush). I left the
primed boxes for a couple of days (at roughly 68 degrees F and low
humidity). When I tried to sand it the primer clogged the paper, and came
off in little rolls - basically it hadn't fully dried and was still slightly
plastic.

I (tediously) sanded all the primer off with a powered sander, and bought
another batch (with a different batch code).

This time I used a roller, then left the boxes in a drying room (around 80F)
for a week. Guess what happened when I sanded it again? Yep. Same result.

I did contact International to work out what went wrong. They sent me a
catalogue. Thanks.

What was the primer you've seen?

Sploo.


That rather depends upon *which* stuff. International make a primer
that is the best I have ever seen (for your purpose). It was originally
formulated to fill the grain on Phillipine mahogany in marine
applications...dries fast, sands very easily. Don't recall the ID# but
the last time I checked (couple of years) it was running about $80/gal.