Poor quality MDF

[Minwah]

I was lucky enough to get (from a mate ) some really nice quality MDF for the bulk of my cab...

Unfortunately, I did not have enough for the whole thing, and had to resort to buying(!) some from my local DIY store.  The color is different (darker), and looks a lot more grainy/bitty, but I didn't see this as a problem initially...

...but now I have primed/sanded and top-coated a panel of it, it looks awful    The grainy bits show up really bad (the good MDF looks nice & smooth painted).  What should I do?  I have made one panel out of the crap stuff that I REALLY don't want to have to make again - should I take a power-sander to it?  Or try surface filling it?

Any suggestions would be great...

[CitznFish]

laminate it.

[Minwah]

laminate it.

Trouble is the cab is >half built, and I don't want to laminate the rest of the machine (probably not even possible now).  Plus, the panel in question needs screws thru to hold it on.

Thanks for the suggestion tho

[Spaced Invader]

Textured paint?

[Minwah]

Textured paint?

I don't think it will cover it.  I am painting it using a roller, and on the good quality MDF all you can see is the pattern from the roller (I like that).  On the poor MDF there are bits quite sigificantly 'sticking out'.  It reminds me of the crappy bits of woody-stuff that you get in cigarette tobacco sometimes.  I think even textured paint will not really cover it up.

I should add that my top-coat is not yet dry, so perhaps it will not look as bad tomorrow  

[CitznFish]

thin sheet of wood veneer, prime and paint.

[spidermonkey]

You didn't buy partical board by mistake did you ? Because that stuff looks like $hit when painted. Even mdf isn't really made to be painted but I would get some Nitro Stan/Nitro cellulose or some spot putty used in auto body work. Slather on a thin coat and dry sand it with 220 and then 320 grit. Use the green spot putty from 3M. It doesn't shrink like some of the cheaper spot puttys so you may get by with a single application. Don't use regular body filler as it is too thick and won't fill the little crevices.
Good luck Minwah.

[ErikRuud]

I sounds like they sold you something other than MDF.

MDF is made from very fine wood fibers, almost like a powder.

Take a look at this page to see the difference

[Minwah]

It is definately MDF (I've used chipboard etc. before), I'll try to explain a bit better...

If you run your hand over it quickly, it feels smooth as normal.  The 'bits' are only apparent when very slowly running your hand over it (they just feel like very slight bumps).  Even after priming, I could still see the MDF (thru the thin primer), although it didn't look bad.  Having applied the top-coat, the 'bump's are clearly visible.  Wierd.

It is as if the powder that it is made up of had some bigger bits mixed in with it every so often.  I don't have a close up photo atm, and from a distance it is impossible to see.

I don't know where I would get veneer, so I might try sanding it.  I am not sure if filling will work as there are relatively few high spots (bumps) in comparison with the 'normal' surface.  It doesn't have a pitted surface like chipboard.

Thanks guys, I'll see how it goes...

[Bonesthedog]

Start off sanding with  a rough grain of sandpaper (80) and move up to about 150. That should get it al even. Ive had this problem with real wood before. Soung like chunkier fibers got in with the mdf.

Dont sand too much.

[WiZeR]

out of interest minwah, where did you get this stuff from?  Might prevent fellow projecteers buying it in the future

[Minwah]

Start off sanding with  a rough grain of sandpaper (80) and move up to about 150. That should get it al even. Ive had this problem with real wood before. Soung like chunkier fibers got in with the mdf.

Dont sand too much.

Thanks for the tip

I got the stuff from Focus (a UK DIY chain).  I expect the quality of the stuff varies, but I have bought 2 or 3 batches which seem more or less the same...

It doesn't look any better with the paint dry btw.  Hopefully a bit of elbow grease should make it ok

[_Iz-]

You need to skim coat it with autobody filler or spray it with a high solids primer and sand smooth. You could probably skim coat it with drywall compound as another alternative (and sand smooth) Try not to sand through the coating you put on.

[Hammerbot]

Yeah, you need to fill the cracks and pores of the lower quality MDF.

A sanding sealer may also do the trick. Fill it in, smooth it out and prime and paint. Should not be a difference.