Priming MDF Mouldings
A solvent-borne primer may reduce MDF's surface fuzz after machining, but some sanding is almost always unavoidable. January 22, 2006
I manufacture MDF mouldings of all types. My problem is getting a smooth, primed finish without having to sand. Is this at all possible?
(Architectural Woodworking Forum)
What type of primer do you use? Latex base tends to raise the grain in wood products where an oil base would not, however oil bases take more time to dry and that can cause other problems, depending on your operation.
Having finished lots of MDF, my answer is Not Possible. This topic has been covered extensively in the Finishing Forum. Try a Knowledge Base search.
To get smooth profiles, use your thumb or a plastic putty knife and apply drywall joint compound to the fuzzy profile material exposed during the routing. You'll have to lightly sand the joint compound with 220 grit paper. In general, the best way to keep the fuzz down is to prime your MDF with shellac and then paint over it with good quality paint, but you'll still need to lightly sand with 220 grit paper to remove the fuzz. There's no way to get around a modicum of sanding, though.
We (Courmatt) have provided a number of firms with a PCD cutting edge. It has reduced the amount of sanding, if not eliminated it completely. PCD loves this type of material. It is not cheap, but designed correctly, not only do you get a superior edge, but over 100K in linear footage. If you are doing only a few thousand feet, PCD will be too expensive, but if you are using a certain profile frequently, this cutting will decrease tool costs.
On finger jointed pine, manufacturers now use what they call "clay coat," and it does have a better surface than the first generation finger jointed trims. This may help with MDF. 99% of finger jointed moldings are radiata pine from the southern hemisphere. They all are using the clay coat as their sales pitch. It seems like a heavier primer, not a thinned down product like the first generation. Almost like you have a thick topcoat applied.
Only one thing can help you - lacquer the moulding first and then you will have no problem of hairs. I have done it and it's quite successful.