Painting MDF

[gad5264]

Good morning all, I am looking for information on the best sealing practices for MDF before I spray paint it. I cut some letters out this weekend and am placing them on a sign and want to spray paint them. I am assuming that the edges will be like sponges and suck up the paint.

Any thought and insight?

[fredhargis]

Is this an outdoor sign? Anyway, the edges will suck up paint but if you seal them first with a good primer (I prefer an oil based version) you should be just fine. It wouldn't be a bad idea to use the primer over the smooth surfaces as well.

[Cooler]

Only the machined edges require special attention.  The factory surfaces take paint beautifully.  If you dress the edges with Sealcoat it will dry in 20 minutes and you can then apply the primer. 

Putting a waterbased paint over the machined edges will allow the fibers to expand and it will have the texture of 40 grit sandpaper.

If you want a very smooth finish on the machined edges then you should add filler to the surface before the shellac.  I like to use Parks' grain filler (usually used for oak flooring) as it dries very quickly, sands easily and takes paint well. 

Hand sand the machined surfaces.  Use the grain filler like shoe polish and rub it into the pores with a rag.  Wait 30 minutes to 2 hours for it to dry and then hand sand again.  Then seal with Sealcoat and then prime and paint.

Plaster joint compound works too but dries slower and sands messier.  I would suggest that MDF is an indoor product only (and not to be used in bathrooms or places with lots of moisture).  MDO is designed for exterior applications.

[gad5264]

Thanks guys, this is an indoor sign for an office.

[jteneyck]

I use joint compound for the edges because it's cheap - and works fine.  I prime everything with BIN white pigmented shellac based primer before painting.  In addition to sealing the surfaces it also helps cover the brown color of the MDF so only two coats of paint are needed to get the true color.  

John

[Cooler]

I use joint compound for the edges because it's cheap - and works fine.  I prime everything with BIN white pigmented shellac based primer before painting.  In addition to sealing the surfaces it also helps cover the brown color of the MDF so only two coats of paint are needed to get the true color.  

John

I would note that some of the acrylic finishes are not happy to go over shellac.

PPG's Breakthrough will craze over shellac.  I have not tested other acrylic finishes.  So testing might be a good idea.  As mentioned in my earlier post the joint compound works fine.  It is slower drying and messier sanding than the grain filler.  A tub of grain filler will last a very long time.  Especially if you wrap the lid with Saran wrap to keep it "fresh".

[jteneyck]

I would note that some of the acrylic finishes are not happy to go over shellac.

PPG's Breakthrough will craze over shellac.  I have not tested other acrylic finishes.  So testing might be a good idea.  As mentioned in my earlier post the joint compound works fine.  It is slower drying and messier sanding than the grain filler.  A tub of grain filler will last a very long time.  Especially if you wrap the lid with Saran wrap to keep it "fresh".

That's why it's always wise to test out the entire finishing process on scrap.  I personally haven't had any compatibility problems yet, but I know it can happen as you pointed out with regards to Breakthrough.  

John

[Cooler]

That's why it's always wise to test out the entire finishing process on scrap.  I personally haven't had any compatibility problems yet, but I know it can happen as you pointed out with regards to Breakthrough.  

John

Breakthrough 250 is a terrific cabinet paint.  It allows recoating in about 20 minutes and it is ready to install in 4 hours.  And no "blocking" (sticking together of painted surfaces).

I have to special order this.  The local stores only carry  Breakthrough 50,  a lower VOC formulation.  PPG is not recommending Breakthrough  50 for cabinets.

[jteneyck]

Breakthrough 250 is a terrific cabinet paint.  It allows recoating in about 20 minutes and it is ready to install in 4 hours.  And no "blocking" (sticking together of painted surfaces).

I have to special order this.  The local stores only carry  Breakthrough 50,  a lower VOC formulation.  PPG is not recommending Breakthrough  50 for cabinets.

I haven't tried Breakthrough yet but likely would if there were a PPG outlet nearby.  But I recently used SW's Emerald Urethane Trim Enamel on some kitchen cabinet doors and drawer fronts and it's amazing stuff.  It's a WB urethane alkyd oil, similar in many ways to BM's Advance, but dries and cures a lot faster, though not as fast as you mentioned for Breakthrough.  It sprays beautifully and flows out great, just like Advance, ready for recoat in 4 hours.  It's available in all of SW's colors and sheens.  

John