On Wed, 1 Jul 2020 20:29:22 +0100, Andrew
wrote:
[snip]

There are 'special' MDF primers, which presumably don't raise the
'grain' (or the fibres in the case of MDF). If you have already
allowed some of it to get wet, maybe a small tin of ronseal
wood hardener might be appropriate before using a primer.

The MDF is in surprisingly good condition considering it suffered from
a leaking bottle of some chemical cleaning agent that dissolved the
paint. It is now dry and solid.

I am happy to join the two metre queue and buy a tin of MDF primer but
before doing so I just wondered what would stop me using the aluminium
wood primer I already have (and will be using on some wood), which is
solvent based. I know water is to be avoided.