Here's my approach, since you're using MDF panels, I would sand the rails/stiles prior to assembly since the MDF won't get sanded. I would sand to 180. I think 1/4" MDF would be fine for the panels, and I would glue them in to make a stronger door. To paint I would prime with Zinnser BIN (the real stuff) smooth that out with a light sanding, then paint. Raised grain shouldn't be a problem, but if it is raised slightly after the prime coat, it will smooth out and you're good to go.

 

Here s my approach, since you re using MDF panels, I would sand the rails/stiles prior to assembly since the MDF won t get sanded. I would sand to 180. I think 1/4" MDF would be fine for the panels, and I would glue them in to make a stronger door. To paint I would prime with Zinnser BIN (the real stuff) smooth that out with a light sanding, then paint. Raised grain shouldn t be a problem, but if it is raised slightly after the prime coat, it will smooth out and you re good to go.

- Fred Hargis

Perfect. Thanks. I had planned on gluing the panels in, and I have the BIN already and was planning to do that. Thanks for the confirmation on those as well. Thank you!

 

The problem with pre-sanding is if there is any misalignment in the cope and stick joint, or face frame you will be sanding to flush them up, or remove glue squeeze out anyway.

Sand after assembly, and I would hit all sawn edges with a knife of some sort (plane, planer, jointer etc) at the very minimum sanding till smooth.

 

You're going to get a different opinion from me. I sand the MDF, especially if the type that has a very smooth glassy type surface. It only takes a few seconds and gives me peace of mind. Since MDF can vary a bit in thickness from lot to lot I check the fit prior to glue up. I've had some fit loose, some perfect & some too tight - you need to know this BEFORE glue up!. If there are any gaps in the panels I caulk them.

Gluing them in is fine, but not necessary. I don't do it b/c I don't want to have to deal with any possible squeeze out. I cut them 1/16 short. 1/4" is fine except maybe for very large doors. 1/2 can be a bit heavy. If you think 1/4 is too light, I would use 3/8.

FWIW, I've painted a lot of MDF doors with BIN shellac primer and so far haven't ever seen a problem, but recently have been told by two experts not to use BIN on raw MDF. I still don't understand that b/c shellac sticks to just about anything even glass.

Sand the frames after glue up. You're going to have to do it anyway to flush everything and get rid of any glue residue. I start with 80 grit and stop at 180.

All edges should be sanded. I don't like to do them by hand or with a sander. I use a 180 grit sanding disk in my table saw to sand edges. I round over with a 3/32 bit.

It's a good idea to sand the profile edges prior to routing. You're probably not going to get perfectly square doors. I leave rails and stiles a bit over sized after glue up I square them up and on a panel sled.

Also, when routing the profiles (grooves) it really helps to do a scoring cut this will all but eliminate tear out.

 

I have a feeling you will find S4S may not be that smooth… It's run through a planer, not a drum sander. It might not also be 3/4… The lumber yard I buy from machines to 25/32, which gives you a touch of room to make sure things are flat and smooth.

I sand after assembly. If I had a drum sander, that might be different.

I agree, 1/4" MDF is fine for panels (not just light use in your family room, either. LOL). Although, I will say I never glue my panels in. I use space balls. I realize that the interior size of the door frame will not change, but it's probably just habit now.

 

Oh, and BIN primer is still a favorite of mine, as well!

Robert says he sands the MDF. I am not sure it really needs it. The alcohol in the BIN will raise the grain of solid wood, but also swells the surface of MDF (very slightly) as well. I find this gives the surface enough tooth for BIN to stick very nicely.

 

The alcohol in the BIN will raise the grain of solid wood

- Axis39

That's odd. NGR stains and dyes are based on alcohol.

 

If you're not going to glue the panels in use space balls from Rockler to keep them from rattling.

 

Also, since this is S4S poplar, and I m priming and painting, I m also assuming I don t need to go through all the grades of sandpaper and don t need to wet between grades to raise the grain. Is that a poor assumption?

- KvG

While sanding surfaces that will be primed and painted is less critical than for clear finishes, you still need to do adequate sanding to remove any milling defects such as the slight "scalloping" you see from commercial planers. And, progressing through the grits actually makes your sanding easier because removing the scratches left by one grit is easier and quicker if you use the next finer grit rather than skipping grits.

Wetting the surface to raise the grain and then sanding is usually reserved for the last pass prior to applying stain or the first coat of finish. I don't think that there is any advantage to doing this between grits. Since you will be priming the surface before painting, I think you can skip this step. The BIN primer will "freeze" the wood fibers and they will be "cut off" smooth as you sand the primer. I have always gotten great results sanding the BIN lightly with 220 grit prior to paint.

 

KvG, As you can see you can get varied opinions, and none of them are wrong. There are many ways to do things so you have to find which one works best for you. The following are my opinions gained from my experience so take them with a grain of salt:

1. S4S boards will vary in thickness from board to board and even within the same board so stock preparation is key. Make sure it is the same thickness and is square and true. This will alleviate a lot of frustration down the line, but is not always possible if you aren't equipped to do so, so just do the best you can.

2. I usually sand after assembly, but sometimes it makes sense to sand before. It's easier to sand the inside profile of your door rails and stiles before assembly, but you need to be cautious not to over sand where rails meets stile.

3. I tend to get tear out when routing the long grain of a rail and stile. To prevent this, pay attention to the grain and rout with it. Take multiple light passes instead of hogging it all out with one pass. I've even gone as far as leaving the boards about an 1/8" - 1/4" wide so if I get bad tear out I can rip it off with the table saw and then pass it through the router 1 final time. I'll then cut to final width on the table saw. For Inset doors leave them wide and cut to fit after assembly.

4. 1/4" MDF is perfectly fine for your door panels. My experience has been their thickness varies widely from sheet to sheet so bring a sample rail or stile with when purchasing to find the sheet that fits it the best. I also like to sand with 220 to clean it up and I always pre-prime the panels before assembly. The BIN primer will most likely "raise the grain" so to speak so you might need two coats with some light sanding in between. If the top coat is a darker color, it is good practice to paint the border of the panel before assembly to prevent any reveal lines during seasonal wood movement. Can you glue the panels in like some say? Well I did on the built-ins I made last summer, but it went against my instincts so I am still nervous about it. A little dab will do ya so use it sparingly to avoid squeeze out. So far no ill affects.

5. You can probably get by sanding up to 180 and then use 220 or even 300 between coats. I would sand all sides regardless of if it is seen or not, but that is me. You do what ever you feel comfortable with.

Good luck

 

FWIW, I ve painted a lot of MDF doors with BIN shellac primer and so far haven t ever seen a problem, but recently have been told by two experts not to use BIN on raw MDF. I still don t understand that b/c shellac sticks to just about anything even glass.

- Robert

Robert, what did they recommend you use then?

 

The person I talked to at Target Coatings told me that about BIN. Also I think their paint is not compatible with shellac.Struck me as funny because in the FAQ's they list BIN Shellac as one of the three recommended.

Just for 100% compatiblity I'm suing primer product they sell.

 

Really? I've used their products a few times and used BIN for first coat, followed by their HSF5000 primer, followed by their top coat of either EM6000 or EM 6500. I did run into some issues with the EM6500 not curing properly on the built-ins I made last summer, but I think it is a product defect and not a primer problem because I only used the BIN for first coat on the MDF panels before assembly.