Another thought for material like MDF (I am not familiar with HDU - we dont see that in Canada under that name. However for mdf one solution you can try is to get some 'wood hardener'. Such a product is available from Min Wax. this is actually designed to be used on semi rotten, or soft corky type of woods. It is some type of plastic product, but it has the ability to soak into the wood and when it cures you have a very hard durable surface that should be good for routing where you need strength and rigidity.
I dont know if this is suitable for exterior applications or not but checking on the MinWax website may reveal more information. I have some of this stuff although I have never used it yet, so I am not speaking from experience, but this is exactly the type of application this product is designed for.
Another product that might work is a substance like 'Cactus Juice' or similar products that people are using for infusing soft wood for turning pens etc. This stuff as well as the Minwax product are basically 'wood stabilizers' and the liquid is a very water thin material that soaks into the wood and then needs to be cured. MinWax, I believe cures on its own, but the Cactus Juice or similar products need to be baked in a low temperature oven, so that may not work for large projects.
Here is a link to a product similar to Min Wax -
http://www.colonialmedical.com/rotted-w ... -2615.html - This material appears to be suitable for outdoor use.
Minwax link:
http://www.minwax.com/wood-products/mai ... d-hardener This also appears to be outdoor safe.
Pete