I am building a laundry basket out of MDF. I need to paint it. How should I proceed once the basket is glued together? (Sanding?, priming?, painting?, other?)
Sanding?, priming?, painting?, other?
break sharp corners: Sharp outside corners are easily damaged, and they don't hold paint as well as not-so-sharp corners. You can "break" the corners with a sanding block or just some sandpaper held in your hand. A router with a small-radius roundover bit works really well -- I use a 1/16" radius bit.
prime before painting: The smooth faces of MDF can be painted without a lot of preparation, but a coat of primer will seal the surface and give the final coat a smoother finish.
There should not be much to this. One of the main advantages of MDF is that it is paint ready.
There are still some things you need to consider where the drawbacks of MDF come into play. Rob brings these up in his comments so I mention them here at an attempt of completeness.
- Weight - MDF is very dense which makes it heavy. Depending on the thickness you could end up with one heavy load. (Nothing to do with painting of course but still worth mentioning.)
- Water absorption - A couple of points in this regard. If you have laundry in there and your MDF is not sealed against that it will make the MDF swell. The is more important at the edges like where you might cut handles or near joinery.
Leading from that I would only suggest that you prime if you plan to use a lighter colour and don't want to have to worry about MDF brown ruining the look. If you do prime make sure you don't use a water based primer for reasons mentioned above. If your actual paint of choice is water based based then priming would be a good idea!
There are also suggestions about sealing the edges of MDF which should be considered. This might also come into play if your basket is more that just a simple box with handles. Using a mud or spackle is recommended since, like end grain, the edge of MDF will absorb paint irregularly. Since it is likely that moisture will come into contact with this project you should do what you can to protect it. Covering all exposed edges and after it dries just sand it down.
Should be a good start to a great laundry basket I think.
Good advice here from Matt and Caleb.
Just thought I'd tell you how we do it where I work:
We use MDF all the time, including moisture-resistant MDF for external boarding. We also sometimes run V-grooves or flutes into the faces of boards for a decorative detail.
Before painting, we sand all faces, paying particular attention to cut edges (so the edges of boards or any cut-outs), also making sure to sand a small (~0.5mm) chamfer to any sharp edges. Then we seal the cut edges with standard timber end-grain sealer (but you could probably use PVA glue), then paint with a primer, nib and paint with topcoat. This gives a good smooth finish every time.
I would not skip the edge-sealing stage, if you're going to have any of the edges visible, as if skipped this will give a very rough-looking edge and will absorb the paint inconsistently. A good coat of primer might do it, but it would depend strongly on the type of paint/primer you're planning to use.
You can prime and/or paint the faces of MDF immediately if you wish. Sanding is not strictly necessary, although it can help primer or paint adhesion, will remove surface contaminants as Graphus mentioned in a comment below, and can smooth out small dents and scratches as WhatEvil mentioned.
As Matt and WhatEvil mentioned, it's also a good idea to seal the edges (e.g., with with drywall compound) before priming and painting.