Do you sand your MDF?

[Craig D Peltier]

My finisher says I need to sand it. If not when applying primer and paint you can see the scratch marks form the sander.
Do you'll sand your MDF?

[Scott Holmes]

If you have scratches on the surface then yes. I will usually sand any profiles routed on the edges.

[Jeff Jewitt]

Yes - but I use higher grit sandpaper than I would on "normal" wood, typically 320 or 400.

[Prashun Patel]

i made a bunch of mdf shelves for my closets. I never experienced scratches. I did get a smoother paint job by sanding in between priming BIN coats.

I didn't sand the raw mdf - only the machined edges. They can absorb paint to no end. The BIN primer did a good job of sealing the edges too.

[glenn bradley]

I also use 320 to 400. With this grit my ROS does not leave swirls. I am careful to clean off the surface while working so foreign matter doesn't make scratches. I hand sand as well just like with any surface. I will use even finer grits on the ends when I finish with shellac to sort of burnish these surfaces so they don't soak up so much.

[Phil Phelps]

My finisher says I need to sand it. If not when applying primer and paint you can see the scratch marks form the sander.
Do you'll sand your MDF?
What grit are you using? As others say, you don't sand the face, just where you cut, or profile.

[Craig D Peltier]

What grit are you using? As others say, you don't sand the face, just where you cut, or profile.

Im talking on flat surfaces. He had some primer on it and you could see the factory plywood sander lines in it. Looks like 80 grit or so. I didnt sand.

I know on the edges he used to use spackle smeared on to seal it, now he says he just sprays primer on it till its almost dripping , then cleans it up after dry.

[Phil Phelps]

I've never seen that from good MDF. Anyway, you know how I finish the stuff.

[Prashun Patel]

I used the drywall compound trick on edges. It works, but it's messy, and it can be brittle if it gets rubbed. Sealing with a primer is better IMHO. Use the Zinsser BIN primer if yr not adverse to non-water borne primers. The BIN contains shellac which is an excellent sealer. It also dries nice and hard so when you sand with 150 or 220 in between coats, it'll level brush/roller marks and will level any dust.

To that end, if you have some regular shellac onhand, you can even use that to seal the edges. To be safe, I'd use dewaxed, though if yr planning on latex on top.

[Phil Phelps]

I used the drywall compound trick on edges. It works, but it's messy, and it can be brittle if it gets rubbed. Sealing with a primer is better IMHO. Use the Zinsser BIN primer if yr not adverse to non-water borne primers. The BIN contains shellac which is an excellent sealer. It also dries nice and hard so when you sand with 150 or 220 in between coats, it'll level brush/roller marks and will level any dust.

To that end, if you have some regular shellac onhand, you can even use that to seal the edges. To be safe, I'd use dewaxed, though if yr planning on latex on top.

I do not use compound on the edges, Shawn. Bin pigmented shellac is much better for the edges. It does take a while to dry enough to sand, however. If you have two to three or more square feet to finish, try the joint compound and see if it doesn't work much better than your sealer. You don't pile it on. You trowel it clean and then let it haze over. 220 sanding and then prime. Great results.

[Steve knight]

the fast and easy way to seal mdf is to thin down yellow glue and brush it on. I thin it down just enough to make it brush able. plus it also hardens the edge.

[Scott Holmes]

BIN shellac is MUCH harder than yellow glue. Yellow glue tends to be a bit flexible. Shellac on the other hand is VERY hard.

Under paint it probably does not matter.

[Steve knight]

as a surface treatment. but it takes several coats of shellac and could take several days to dry. yellow glue is ready in the morning or earlier and only takes one coat. I work with a lot of mdf and do a lot of signs and such out of it. glue is fast and works in one coat.

[Scott Holmes]

Steve,

Shellac dries in 10-15 minutes (30 max), MUCH faster than watered down yellow glue.

Where and when have you see several days for shellac to dry?

[Steve knight]

the first coat does but each coat takes longer. but when it soaks into mdf then it takes far more time to dry. I have sealed a lot of mdf as I cut tons of it. shellac sucks for it and it is slow to dry. the yellow glue soaks in a bit but it dries pretty fast and only takes one cat to make mdf proof against airflow.
thick primers work but they usually take two or three coats.

[Scott Holmes]

Interesting, I have not used that much MDF; I like wood, not MDF; I have used it for jigs etc. I never really tried to seal it more than one coat of shellac.

Thanks for the info...

[glenn bradley]

My 2lb cut dries in about 20 minutes regardless of coats. The DNA gasses off and the solids are left behind. I've never had any for more than about 6 months before replenishing. Could the slow drying stuff be old?

[Steve knight]

Interesting, I have not used that much MDF; I like wood, not MDF; I have used it for jigs etc. I never really tried to seal it more than one coat coat of shellac.

Thanks for the info...

not my favorite stuff but I cut a lot of it on my cnc router. made a 11' guitar out of it.
but so far yellow glue seals and lets you sand the edge smooth fast. about the only one coat solution I have found.
I have been making a lot of molds for sign plastic forming from it this week.