Sealing, varnishing and finishing MDF

No PVA.

Seal it with a couple of very thin varnish coats.
Then smooth/denib when dry. One more thin coat, smooth and then top coats.

Mr. HandyAndy - Really

 

Kram,

You can buy a proper MDF sealer. Its very, very runny and looks like watered down milk. When its dry, it turns clear and seals all the pores in the MDF. It may just be a type of PVA, but PVA has a sweet, distinctive smell and this stuff doesn't smell anything like PVA.

A few years ago, I built a cabinet out of MDF and I wanted to paint it with Black Hammerite. The first couple of coats just sucked up like a sponge. In the end I remade the cabinet and sealed the MDF with this sealer (2 coats) then one coat of the Hammerite and got a perfect finish, just what I was after!

 

I suspect the proper MDF sealer acts a bit like masonry sealer - watery as Snezz says, to soak in as much as possible but then dries to act as a barrier to further absorption.

Why not PVA? Because it'll reactivate if allowed to become wet, and can also do so if you were to use water-based paints on it afterwards. If you were to roller emulsion paint over PVA, you'd find that it starts to go on nicely, but you then end up with it dragging, and probably becoming very 'textured' as the PVA becomes an adhesive again...

As Mr Ha says, you will also need to denib after sealing and before finishing coat. That's just using a fine grade abrasive paper - 180 grit or more? - to remove any raised fibres especially on the end 'grain'.

 

MDF primer will almost certainly be opaque white, so only suitable if you want to paint it afterwards as opposed to clear varnish.

Will your computer desk be painted? If so, easy-peasy. 'Cos I made an MDF pooter desk for my in-laws a couple of years ago from MDF.

To paint it, you do indeed need 'primer', and do yourself a favour by using quick-dry which will be water-based.

How to apply really depends on what final finish you want to use - I wanted a clean, smooth, durable satin finish for the top in a white, but matt 'magnolia' for the sides to match their walls. So, the sides (in fact the whole outside 'carcase' of the unit) was simply emulsioned using the same paint as their walls - matt emulsion will go straight on to MDF. The only 'issue' was that the panel edges (which were double-thickness for appearance) needed flattening down with 180 grit paper before a final coat. I applied the main panel finish using a brush as I'd also routed vertical 'V' grooves in the panels for a bit of interest.

For the top desk surface - which was 12mm MDF with a heavy 'dado' moulding fitted along the front and sides (mitred, glued and pinned) to add 'depth' - it was a simple case of using a mini-roller (I think 'foam') to apply an even coat of primer, allowing it to dry (very quick) and then flattening it down with 180grit around on a sanding block.

This will probably need a second coat.

Anyways, once your surface is primed and flat, apply your top coat using the same foam mini-roller. I used white satinwood which is water-based. Level out the paint using the roller and finish with very slow rolls so's as not to create bubbles in the paint. If you fit a front rail like me, then finish this off using a brush instead, going with the grain - it just looks better.

When the paint dries, it'll hopefully have the very finest of texture, very much like a 'manufactured' finish. Two coats would be good, with the lightest of de-nibs in between (don't sand it heavily or the paint will likely 'tear' rather than lose its nibs...)

The only snag with water-based satinwood is that it does tend to remain tacky even after it's fully dried - there are threads about this on here. You place solid objects on it a couple of weeks afterwards and they 'stick'!

However, this effect does go away, and can be improved by spraying it with normal furniture/multi-surface polish and wiping it down after, say, a couple of weeks.

 

Kram, I cannot comment on 'proper' MDF sealers as I've never used them. However, I can't help thinking that any 'clear' sealer would still darken the MDF as if wetted?

So I'd be personally inclined to skip the sealer and use a good quality water-based varnish of whatever finish you'd want to end up with - matt, satin or gloss. Something like Dulux 'Diamond Glaze' floor varnish; http://www.duluxdecoratorcentre.co.uk/servlet/ProductHandler?code=DDC10058

This will dry quickly, leave a non-rubbery finish ( ) and be very hard-wearing.

But there are plenty other makes out there too.

 

I haven't used it but the Rustins Mdf sealer states that it dries clear, personally for this type of application I'd use a clear shellac.

 

I was scrolling through this post to say use shellac sanding sealer and RWC beat me to it. Sand the edges with fine wet and dry, about a P400 or finer, seal, re-sand and then seal again. It will be as smooth as the face.

 

Sigh - do as these two coves say^^ .