How much moisture damage is too much for MDF?

[Mr Kray]

Hi Guys,

Just a quick question.. How much moisture damage is too much for MDF?

I did a quick search and found some generic answers but wanted to know specifically for my cab as I'm buying more MDF today and want to know if I should be replacing some of the already cut panels.

I'm basically making a low-boy cab, sort of an upright-side-by-side-cocktail hybrid, 'cause I'm lazy and don't like to stand. It's all 18mm MDF but - and don't kill me for this - it's been sitting in a carport for the last 4-5 years! 

It was covered with a tarp but it was also near the edge of the carport and has been through many rain storms with water falling near the cab (half a metre) but not actually getting the cab wet directly (it's on small wheels).

Now I've comeback to complete the project I've noticed that the MDF has expanded by about 1mm or so in certain parts and there is some slight surface mould in the inside of the cab (the mould is literally just on the surface and scraps off easily, it's not set in).

Now is that 1mm going to be a problem? Does this mean the fibres within the MDF are now loose and it will start to degrade further? should I just sand it down slightly to even up the joints and make sure no more moisture gets to it?

The front and base are easy to replace (but I would rather not pay for more MDF than necessary) but the side panels took a bit of working out and routing to get right and it would basically mean I'm building the cab from scratch again.

Any help would be much appreciated.

Thanks,

Mr Kray

[sstorkel]

MDF is so cheap where I live that I wouldn't bother trying to save it. Of course, I think MDF is awful stuff and I'm always looking for an opportunity to throw it in the trash...

Is the wood absolutely, completely dry at this point? Will the humidity in your area ever be lower than it is now? The problem with sanding it down is that if the wood isn't completely dry it may shrink in the future. So you'll go from having a perfectly flat surface to having a dent. Remember that wood also expands and contracts as humidity changes. If you sand today and the humidity is lower later in the year, you may experience the same problem. Painting all the sides and edges will help to mitigate this problem, but may not remove it entirely.

I think the big question is: do you want a perfect cabinet? Or do you just want to get it done? If you want perfect, get rid of the damaged MDF (and replace it with plywood  )...

[Franco B]

As sstorkel said you would be better off buying some fresh material if you can afford it. You shouldn't need to buy/use too much if you are building a low boy (that depends if you dont mess up, I ended up buying two more sheets when i built my first cab  )

Save the damp MDF for a project where the finish isn't as important and treat yourself (and your cab) to some new material, you will only regret it in the long run if you don't.