primer for plain MDF, is it actually required?

[Joe90]

Hi

I'm wondering if I need to use a primer before spraying raw MDF with pre-cat lacquers in a solid colour.

The primer I have is a hydrocarbon free one from Morrells... it is very thick and needs to be thinned a lot (30% - 40%) to spray through the 1.6mm nozzle. (Maybe 1.8mm nozzle)

If I just spray the precat lacquers straight onto the MDF what would happen? Is it bad? Or does the primer just provide a good base for the topcoats, meaning less topcoats are required?

Cheers
Joe

 

[jasonB]

Scrit is the best one for this but you do need the primer and it will require flatting back before any top coats can be applied. You will need a lot of the primer on cut edges.

Jason

 

[George_N]

I've recently been spraying MDF and if you don't seal it it will soak up the paint like a sponge. I seal cut edges with PVA diluted 5:1 (PVA:water) before the primer.

 

[Scrit]

As Jason and George say, if you don't seal MDF it is like spraying blotting paper. The problems are that there is potentially oil contamination of the MDF surface off the pressing platen, that the contamination can be inconsistent (resulting in a patchy effect), that the edges absorb at a much higher rate than the flate surfaces and that anything sprayed will tend to raise the grain, particularly on the edges, and more especially if the finish is water-based. I've tried a few techniques to get a good finish in the past, but the best approach appears to be something like:

2 coats of MDF sealant on surfaces (4/5 coats on edges) flatted between coats with Scotch grey abrasive cloth. It doesn't have to be MDF sealant, though, as a blonde dewaxed French polish will achieve the same, but dry in minutes in a warm room - the disadvantage being that it costs more, but on the other hand it does seal the MDF extremely well. PVA works, but I find it takes a long time to dry, relatively. This gives you a surface which doesn;t absorb finish like a sponmge or suffer from grain raising.

2 coats of high-solids primer, second coat applied wet-on-wet. Denibbed when dry. This gives you a smooth surface of consistent colour to work with.

2 thin coats of finish, again 2nd coat applied wet-on-wet.

I've never had any success with a single thick coat. They are much more prone to runs and sags, IMHO, whilst two thin coats, allowed to flash off for 15 to 20 minutes (or whatever the manufacturer recommends) should result in a nice consistent finish with less defects and consistent tone throughout.

If you spray finish straight onto the MDF you will use a lot more of it because of absorption, the edges will be as rough as sandpaper and the colour will be patchy if there any inconsistences in the surface of the MDF

Scrit

 

[engineer one]

scrit has much more experience than me, but mdf is very absorbent, and thus does need coating with a sealer. but there is something to be said for "dusting" it on.

when i made model cars, we used halfords undercoat sprayed so that it was just drying as it hit the plastic, this way the petroleum spirits did not eat into the plastic. safe basically for mdf. you need to build up a layer
that's stops the real paint being absorbed. pay particular attention to the edges, since that will basically wick the paint inside.

what i too would aim for is 2 or 3 layers of dusting to build up a thickish layer of sealer undercoat, knocked back and then a proper coat of the
undercoat then use the colour.

thinner coats offer the better chance to get a deep gloss finish if that is what you want, and of course you can use car paints on it after sealing, making it easier to spray. it also allows for a smoother finish on any mouldings etc.

paul :wink:

 

[kafkaian]

I've recently been spraying MDF and if you don't seal it it will soak up the paint like a sponge. I seal cut edges with PVA diluted 5:1 (PVA:water) before the primer.

Thanks for that tip; when doing mine, I was literally cutting back four or five times before getting a satisfactory edge, whilst the mdf primer I used raised it considerably.

 

[Anonymous]

Well I don't know what you lot are up to but for my contract clients I do one coat of primer and de nib, 2 coats for my jobs, ready for hand painting on site. Never had any problems. It is MR MDF though.

 

[Scrit]

Don't you find the end grain a bit of a faff, though, even with a profile sander? I suppose it depends omn the surface quality you're after

Scrit

 

[Anonymous]

Nothing wrong with the quality or else the contract clients would go else where, esp as we are not cheap.

 

[Joe90]

Thanks for all the detailed responses, I have found them very useful and I appreciate the time people have taken to respond.

Cheers
Joe