Using epoxy to help hide seams and harden MDF has worked well for me.
Your questions:1. Applying the epoxy is fine on vertical as well as horizontal surfaces.
YES2. Rolling on with a foam roller is common, but what about the inside corners? Is a bristle or foam brush okay?
The thin foam epoxy roller covers are commonly used, however, I prefer the mohair type roller covers (adhesive rollers). Cheap chip brushes work good. Cut them shorter so they have stiffer bristles. After rolling on the epoxy, tip off with a foam brush.3. 127 resin and 350 non blush hardener works.
There are lots of epoxies that will work fine. I have the most experience with Raka 127/350 building boats, so it is the one I recommend only because I have successfully used it on speaker cabinets.Things I don't know:
Do I need to use thinner and what do I use?
You don't need to thin epoxy, actually it is not recommended as it will weaken the epoxy, but I did for the first coat
so it would soak into the surface of the MDF. Not concerned about weakened epoxy on speaker cabinets. You can thin with alcohol, acetone, or lacquer thinner, but only thin about 5-10% max. Is 1 and 1/2 quarts from Raka as a kit enough to do subs, stands for something like wedgies, and wedgies themselves?
That should be enough.
You will probably need 3 coats of epoxy including the first thinned one if you decide to thin. If you do the roll and tip method, the orange peel effect will be less making sanding easier. You should sand between coats (when hard) enough to knock down the high spots. If you sand through a spot, just reapply new epoxy at the spot. Raka 127/350 is a 2:1 mix by volume, but I like weighing it instead with a digital gram scale. the mix is 100:44 by weight. Don't mix more than 3 oz. at a time, and dump it into a roller tray. Cut your covers to 3" so you get 3 from a 9" cover. You should probably practice on some scrap MDF before committing to the speaker cabinets.
- wear nitrile gloves
- eye protection
- dust mask when sanding
- work outdoors if possible