Thread: Oil based undercoat on MDF drawing Wax to Surface

Yes, best first coat on MDF is either water based primer/undercoat, or in the case of clear, shellac.

So Fred, does this happen reasonably quicky or over time? As over the last couple of years I have painted a fair amount of bare MDF with the first coat being an oil based wood primer straight to the finish oil based coats and there has not been any issues.

When MDF first came out we did extensive testing for the panel board manufacturers.

The results were that any solvent based (oil based would be a misnomer) primer/undercoat created drying problems due to the wax in the MDF migrating through the coating. Not only could this slow the drying for some days, but it would then affect the inter-coat adhesion of subsequent coats.

The 3 best sealers were shellac, lacquer sanding sealer and water based primer/undercoat.

Over the years, the panel board manufacturers have improved their MDF to the point that it is now less of a problem, however for best results, the recommendation you state is still correct.

As an aside, when I made my router table, all MDF, I did not have shellac or a lacquer sealer. So I used a polyurethane based sanding that I did have, and I could not put the second coat on for several days.

Yep I noticed that with the MDF where I was in a rush it seemed to take for ever to dry, but where I wasn't (planning ahead) it dried completely and was able to sand with no clogging.

My reason for this post is that I'm currently making a TV cabinet from plain MDF that is going to be finished in a satin oil based finish. The instructions on the can of Master Prep Multipurpose Undercoat has an additional category for bare MDF in which it states to apply a coat of Aqua Prep first. I emailed the paint company on this to which the teko advised of the above.

However I will I dare say that I will go with what I have ie. the Master Prep followed by the satin finish. I will also sand all surfaces to remove that sheen that MDF has thus allowing the paint to bond. And then wait for something to happen.

So why go to all the trouble of reading the instructions, contacting the paint company for further advice, put up a post here and then blithely ignore it all?

You are one of those customers that would drive use technical people to despair, what's more a fair percentage of those customers that blithely ignored all good advice would then ring up and abuse use if it all went pear shaped.

Amazing

Yep fair comment Fred, but I don't/won't when it my fault, except curse myself.

If you think the first coats solvent is going to affect the MDF then all you need to do is clean the surface with the solvent used in that finish.

You can paint MDF with any paint finish ( I always use Acrylic Urethane but it applies just the same for oil base paints etc) it dissolves the wax the wax mixes with the finish and therefore the finiah can't dry and go hard as its got wax through it.

Use clean rags wet with the solvent, wash a small area while wet wipe off with another clean dry rag. If in doubt repeat process. The straight solvent will penetrate further than the small amount in the paint. You can then finish all the steps without problems. You could sand but its not really necessary as the paint gets its adhesion by soaking into te surface, rough sanding may bring wax closer to the solvent again.

You could use a shellac or sealer but often this weakens the overall finish. For instance a polyurethane with shellac under it would be subject to blistering from heat (hot cups etc) where it would not if only the correct primer etc was under it.