Preparing an MDF Carving for paint

[Underdog]

"Door Grade" MDF will probably be the best bet. We get ours from Georgia Hardwoods in Atlanta, but I'm sure you have a supplier somewhere nearby. It can be a bit more pricey than your box stores, but it's worth it. It machines much, much better.

It also pays to put the primer to it, especially the opened edges and carved areas.

[peep]

But how good does mdf carve compared to wood? Hardwoods are so detailed and do not need sanding. Would mdf not be fluffy after carving, loosing detail upon sanding?

[lawrence]

It takes large letters quite nicely and centerline texts VERY nicely... but yes, there are certainly limitations with MDF. To me, the main advantage is flatness and price.

When you are painting a project afterwards anyway, MDF can do quite nicely (especially when painting a project with textured paint)





Lawrence

[Underdog]

I've carved some fairly detailed stuff in MDF. It sands easily though, so it pays to be careful. I'd prime before sanding, let it dry thoroughly, sand, then prime again.

[Deolman]

I read in one of my wood mags you should mix a 50/50 water/glue solution to seal the freshly cut areas. Brush it on just like paint. If I can re-find the article I will post more.

[Underdog]

The idea is to make the loose fibers stiff so as to be sanded off easily. You can do much the same thing with primer or sealer.

But if you start with a better grade of MDF, you won't have to do so much of that, because it already has more glue in it.

[Underdog]

Lawrence, what brand of oil primer have you been using?

I'm looking at our pre-cat varnish here at work, and it's about $30 gallon. I'm hoping to seal my MDF without so much cost.

And will Acrylic paint adhere to it?

[200k]

Zinnser SealCoat is really just a brand name for dewaxed shellac (note: dewaxed - important!) which is my favorite coating to seal all woods. For the best MDF results I flood the surface and let it soak in. Then after it dries I do it again until it will take no more, especially end grain. This will leave a semi-gloss sticky residue/coating that is hell to sand off. It will clog a sheet faster than uncured glue. But you don't have to sand! Shellac is nothing but bug spit dissolved in alcohol. Unlike most other coatings, it doesn't change the chemical linking when it dries so it will always dissolve in alcohol. So the easiest way to get rid of that excess shellac is to just wipe the surface with denatured alcohol and rags until it doesn't come off anymore. Be generous with the alcohol and rags and of course do it in a well ventilated area - the stuff's poison. Once you get to the flat look, let it dry for 24 hours and it will finish sand without clogging and prevent any further coatings, i.e. paint, varnish, polyurethane, etc. from soaking in.

200k

[Underdog]

I'm going to try the Zinsser BIN shellac primer. It has a fast dry time and has been recommended by the finishing experts over on Sawmill Creek. We'll see how it goes. Since it also comes in aerosol spray, I'm going to try spraying it with my HVLP gun.

[200k]

If you plan to spray, make sure you have tons of ventilation! As I said before, the solvent for shellac is denatured (methyl) alcohol, not the nice friendly ethyl alcohol that you are familiar with in various consumable flavors, but the stuff that, during Prohibition, killed and blinded thousands of people drinking bathtub hooch that had been "stretched" with the still legal methyl alcohol. Since it is water soluble, it is very quickly absorbed through the lungs and into the bloodstream even faster than petroleum based solvents (i.e., glue sniffing) and instead instead of a pleasant drug high you get blindness, peripheral numbness, and permanent CNS damage. Baaaaaad stuff. It might even be prudent to use an organic filter air mask too.

When applying with a brush or rag, good ventilation is sufficient, just as fast, and you don't get overspray.

If I have a large flat area to saturate, I will just pour a significant amount into the middle of the piece and slosh it around with a brush or small rag until it is all absorbed or wiped off. Very quick.

200k