[quote=prueg;649844]

As for painting MDF... some people say you don't need any primer or sealant, except on the edges, where you need to use dry cement or plaster or something (can't remember) as its very thirsty there. Can anyone add their 2c to this?
You have to seal the edges. I use PVA diluted with wather as a glue sixe then sand them again.

I prime undercoat with zinser shellac based primer/undercoat. Whether stricly necessaary I doon't know but it seems to give me better results.

Also, for a storage unit like that, whats the best way to paint it? Spray it? Brush? Once assembled, or prepare all the pieces, paint it, then assemble?
I spray it but the box thing can be tricky if you do it after it is assembled. If you do spray it do not put a back on first you will need the access and the back gives some blowback even with a low pressure gun. It is hard (for me anyway) to get a good even coat in the confined spaces of boxes.

You might prime the lot befor assembly, paint the inside pieces first then assemble and paint the outside last.

Some people get a good result with a smooth roller but I tend to get little dimples in the finish (poor technique perhaps). Brushes are always an option but again it depends on your technique.

I am sure a painter will give you better advice but in any case when doing this project you will need to consider what will be seen and how to get inside the boxes to get an even coat.