Sub box/enclosure sealers/treatments?

Before I begin my question, I just wanted to say thanks to all those on this board. This board has been a huge help and has really convinced me to look into many different options in system building for my new truck.

Ok, so here's the question.
Many years back, while I was in high school, a friend of mine used to make sub enclosures for friends and the like. He would make the boxes out of MDF while using a little of the high school trigonometry we had learned. He'd throw in some insulation material for the finishing touches, and we'd always joke about it being pink panther insulation.

Getting to the point, for our closer circle of friends, he would also treat the box with some kind of sealant after construction. However, he never did it himself. He would take the box to a shop that is now no longer in business, and have the treatment done there. Over the years, the shop owner never revealed the ingredents of the sealant. But, he always called it the "secret sauce."

I can tell you guys this. When drying, 24-48 hours, it would smell like the most awful combination of Thompson's water sealant, mustard, and the most sour smell possible.

So, the reason for this whole story, is the question, does anyone "treat" their sub enclosures anymore? Is that a thing of the past? If not, anyone care to share their findings on a secret sauce recipie?
Thanks in advance!