Sealing a desk made of MDF with decking oil?

[Mattyess]

The problem is you can never really tell what 'out of recommended use' things are compatible till you try 'em.

So you're saying that the poly might stick?

[Master Splinter]

It might stick permanently and very well and you'll never have any problems with it; it might stick initially but start delaminating after a week/month/year, or it might - after a week or two of looking perfect - suddenly revert to being a gluggy, sticky mess which has to be removed with a scraper and the sort of effort only justified by a substrate as expensive as a granite top.

There's no way of knowing - short of immediate and calamitous failure - how the chemistry of the two finishes will react over time.

Even well researched, designed-to-be-compatible paint systems can have problems - just look at your nearest car park and count the number of cars where the clear coat has broken down and is flaking off.

[Mattyess]

It might stick permanently and very well and you'll never have any problems with it; it might stick initially but start delaminating after a week/month/year, or it might - after a week or two of looking perfect - suddenly revert to being a gluggy, sticky mess which has to be removed with a scraper and the sort of effort only justified by a substrate as expensive as a granite top.

There's no way of knowing - short of immediate and calamitous failure - how the chemistry of the two finishes will react over time.

Even well researched, designed-to-be-compatible paint systems can have problems - just look at your nearest car park and count the number of cars where the clear coat has broken down and is flaking off.

Fair point.

Well I decided that a calamitous failure was a risk I was willing to take, as the oil had left the MDF feeling a little fuzzy, only slightly water resistant and any liquid that touched it would stain it. I went to my local paint store and the guy adivsed me that if it can still absorb water, then it should be able to absorb water based poly.

I've just chucked the third coat on it and it looks good. I just have to cross my fingers now and hope that it doesn't do as you mentioned and turn into some sort of hideous mess a day after I install it my studio and rest 5 grand worth of equipment on its surface.