What is the best way to seal the edges of MDF before spraying Polystar (white)?
From contributor A:
The best I have found has been bondo or spackling paste. It takes a little more time to apply but you are 100% sure that it will never absorb humidity or water. Otherwise if I am in a rush or have too many edges to do, I either spray the edges with shellac or thinned lacquer, 2 coats minimum.
There is no reason to mess around with sizing or multiple seal coats. The only problem with this type of solution (polishing then 1 coat of good primer) is that it only works for a square edge that you can machine sand. Forget getting a really clean look on a moulded cope and stick MDF door. These need to be hand painted after sealing to conceal the MDF edge grain. We finally bought an edgebander that applies a paint grade tape and we use this for all non critical edges. It is very fast, requires no sanding and the glue line hides. Don't make the mistake of building up a 1-1/2" MDF edge with two layers of 3/4" material and then lacquering it. The glue line will witness. Also, don't make the mistake of building up a 1-1/2" MDF edge and then filling it with wood filler, sanding the filler smooth and coating it. The coating can blister, peel and fail. Don't make the mistake of building up a 1-1/2" MDF edge and then edgebanding and coating this - for example, a countertop. It probably won't fail but if it does you have to repair or remove the counter.
The edge tape for MDF that saves all the sanding belongs on the edges of panels or parts that will be lacquered but can be easily repaired and taken back to the site. The best solution for 1-1/2" painted (opaque lacquer) edges is either polished 1-1/2" MDF. (Ultralite also works fine and makes book case installation etc. a little easier on the back) or miter folding 3/4" MDF with a little fill at the miter point.