There are a number of methods that can be used to secure MDF cabinets when constructing. One way is a miter joint blade, which is a very strong joint with great surface area for glue up. This joint takes a lot of the fitment issues out of the equation by design. IT is also a joint with a perfect seam that is actually where you would want one. Setup and proper tools to make this joint are vital.
Sealing joints can be difficult but there are a number of methods used to eliminate the issue. With a miter joint, there isn't really an issue, but with butt joints like rabbet edges, the valley method works best. Using a router you essentially trench the joint about halfway through the material and halfway between the joint and the edge, along with the same distance on the opposite side of the joint. Then fill with a good stable body filler, sand and seal. This method ensures no telgraphing through finishes or veneers. This has been used in long term testing, and I have yet to see one done properly telegraph through the finish.
Dust is one of the largest downsides to MDF. Even without formaldehyde, MDF is still a chemically concentrated product, and respirators are not merely suggested but necessary when working with it. Proper dust collection is a must. Also a downside is due to its construction, it literally can peel easily. This is a plus when recessing drivers, but for joints can result in an ugly joint pretty quick. Again, proper tooling, and very sharp tools are a must.
When it comes to subwoofers, with the amount of pressure and air movement, I have always been a big supporter of both glue and mechanical fasteners. There are a number of fasteners that work incredibly well in MDF (powerhead screws are my favorite) and when properly installed and countersunk, as well as filled properly, the cabinets are truly indestructible. Dowels make for great braces in subwoofers, and there are a few designs that are pretty ingenious when it comes to proper bracing. If you look up the stonehenge subwoofer enclosure on diysoungroup.com you will see it. Pretty amazing design and application.
I personally prefer working with plywood most times, and find that when properly dampened it works better for the types of enclosures I make. If I was doing things that required complex contours and smooth transitions, I would definitely use MDF. It sands better and is a better product for that type of application.
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