MDF - finishing?

[bugbear]

I just had occasion to varnish (Ronseal yacht varnish) something made from MDF.

The first coat - just soaked in like blotting paper. Zero build.

For next time, is there some kind of sealing coat I should apply first, or do I simply regard the first coat of varnish as being the sealing coat?

BugBear

 

[Vinny]

The simplest first coat is dilute varnish - cheaper on varnish and does the same job as neat varnish.
If you think of what it actually is, very thick blotting paper is pretty near the actual truth.

 

[karma67]

as an alternative i use thinned down 50/50 french polish.

 

[Vinny]

as an alternative i use thinned down 50/50 french polish.

That is pretty much what traditional sanding sealer is - as the name suggests, it seal the grain (in real wood).

 

[karma67]

yes im well aware of that thanks as i did french polishing for a living.

 

[sq225917]

Mdf is a bloody awful substrate for liquid finishes.

 

[Vinny]

Mdf is a bloody awful.

No need for qualification

 

[divedeepdog]

The trick to sealing MDF is preparation. Sand through the grades finishing with 240 at least.

 

[Fatmarley]

You can get this stuff in a spray can as well: https://www.zinsseruk.com/product/b-i-n/

 

[bugbear]

You can get this stuff in a spray can as well: https://www.zinsseruk.com/product/b-i-n/

That sounds handy. Duly noted.

BugBear

 

[John Caswell]

I have always used 50/50 or 75/25 white spirit/varnish as a sealer.

John

 

[bugbear]

2nd coat (just applied) was more successful, but still not great.

EDIT; 2 hours later, surface still sticky, so it DIDN'T soak in and disappear nearly so much this time.

BugBear

 

[miktec]

For next time, is there some kind of sealing coat I should apply first, or do I simply regard the first coat of varnish as being the sealing coat?

BugBear

For next time: unless it's for a water/liquid sensitive use (e.g. table top) just give it a couple of coats of indoor 'quick drying' varnish with a very light rub down between coats (don't overdo it) . Being water based means it dries very quickly compared to the spirit based stuff so the job gets done in a fraction of the time - added bonus is less stink ....

This will seal MDF much better than any spirit based varnish (unless you apply a zillion coats).

You can then apply as many more coats as you want to finish off - or substitute with yacht varnish if you want a more durable water resistant finish.

Works every time