mdf primer

[eggplant]

Hi all,

I hope this is in the right section.

I am in the process of making something out of mdf, and I got some primer (international MDF primer made by akzo nobel), I tested a bit on a spare piece of mdf and its THICK, and there is no way on earth I can get it on smooth, its about the thickness of gravy. It says to give 2 coats with a ligh sanding inbetween, but by my reckonings its going to need more than a light sanding by any stretch of imagination.
The project I am doing requires several pieces of mdf to be screwed/glued together, should I paint all these first or assemble it all then paint it - the problem with this is that I wont be able to get into the corners and such to sand, but if I paint it first then theres a chance of the finish geting damaged whilst assembling.
Any suggestions most welcome!

Thanks

 

[Scrit]

I am in the process of making something out of mdf, and I got some primer (international MDF primer made by akzo nobel), I tested a bit on a spare piece of mdf and its THICK, and there is no way on earth I can get it on smooth, its about the thickness of gravy.

If this is a water-based compound you could always thin it down first, just like paint. Similarly if it is VOC-based there should be a recommended thinner.

The project I am doing requires several pieces of mdf to be screwed/glued together, should I paint all these first or assemble it all then paint it - the problem with this is that I wont be able to get into the corners and such to sand, but if I paint it first then theres a chance of the finish geting damaged whilst assembling.

I'd assemble first then prime. If you prime everything first you'll need to mask off the joint areas so that glueing is not impaired by the sealer. As for the bits you can't get at, have you thought about hand sanding? If you can't get at them how are you going to finish them in any case?

Scrit

 

[eggplant]

Thanks for that,

Regarding getting into the difficult areas to paint/sand, I figured it would be easy to get in with a paintbrush but not so easy with sandpaper. I dont know if its waterbased or not, I'll check when I get home.

I was actually going to screw it together rather than glue and fill over the screws, is this a bad idea?
Thanks

 

[Scrit]

I figured it would be easy to get in with a paintbrush but not so easy with sandpaper.

You'll probably get sand paper in anywhere you can get a brush, especially if you use small MDF offcuts as sanding blocks and wrap the abrasive round them.

I was actually going to screw it together rather than glue and fill over the screws, is this a bad idea?

The filler will possibly break out or crack the filler over time. The best screws to use are black phosphated carcass screws if you can get them (used a lot in kitchen carcasses, etc). You can get screw covers in various colours, although they do require a spot of glue to fix them and they have a disturbing tendency to drop off (or get knocked off) over time. A better alternative is the knock-in screw cover although they do require you to counterbore/countersink the screw beneath the surface using a tool like this (look for counterbore tools)

If you want to hide the screws another approach might be to use assembly fittings or mod. blocks

BTW, for a hidden fixing method have you considered dowelling?

Scrit

 

[eggplant]

Hi,

Thanks a lot for the detailed reply,

I have seen the screw covers before but on this particular project would be an eye sore. I bought a plug cutter today and then read "not for use on MDF" on the packet. Tried it on a bit of wood and that was a disaster.
I think I'll go with the plastic blocks and some glue, seems like the best option. I did think of dowelling but some of the bits are a bit small and leave no room for error.
Back to the original primer question, it doesnt say if its water based but says to was the brush in water after use so I assume it is, I'll test a bit in a pot.

Thanks again

 

[hermes]

If you are going to paint over it then use 2-pack filler over the (countersunk) screws. As long as it is fully hardened before you sand it flat you shouldn't have any problems. And it's quicker than the other methods you mentioned.

The reason your plug-cutter is not for mdf...I would guess it's made from high-speed steel which will blunt quickly with mdf, but it would really need to be used in a pillar drill to work properly.

 

[eggplant]

Thanks,

Due to the plug cutter not having a pilot or centre point, I assumed it couldnt be used in a hand held drill, I tried it in one of those stands that hold an electric drill - with mixed results....
Can you please give more info on 2-pack filler, I havnt haerd of this, do you have any brand names that I might look for. After much thought, I am definately going to glue and use the blocks but I am going to have to screw in a couple of places and would relly like to hide the screws 100%.

Thanks

 

[Scrit]

You can buy packs of pre-cut tapered wooden plugs as well as strips of taper dowels (sorry, no link). Never seen them in MDF, but then I make MDF stuff using dowels (we gang drill on a machine). For stuff like this, as well as handles, hinges, etc. I'd suggest that as you're near to where you live you might like to get hold of Woodfit's catalogue, after all it's free. They're at Kem Mill in Chorley and are open Saturday mornings.

The 2-pack filler can be something as simple as David's Isopon (car) body filler, although that has a setting time to take into account and you'll have to recess your screw heads a little.

MDF is very abrasive so you really need TCT (carbide tipped) tools to work it.

Scrit