Seam hiding MDF is a challenge for sure. Epoxy can help hide the seams, but I find the flexible (relatively) epoxies are a better choice. The seasonal temperature/humidity changes that cause the MDF seam print through can be helped with epoxy. Which epoxy you choose can also make a difference. Wests system epoxy is a great product, but it is one of the hardest epoxies, almost brittle. An old wood strip canoe/kayak builders trick was to use System Three epoxy (slightly softer) for initial coats, and use Wests epoxy (very hard) for the final coat. In theory the underlying coats allow for some movement without cracking, and the hard final coat adds abrasion resistance. Speaker cabinets don't flex like a boat, so this is probably not an issue. However, I prefer non blushing flexible epoxies with slow hardeners (longer pot life) for all applications.
On my last speaker build a X-CS Encore center channel, I used thickened epoxy for gluing the panels instead of yellow carpenters glue. Then I coated the cabinet with Raka Epoxy (slow hardener) thinned 5-10% with Lacquer thinner. I waited a day, and scraped off any runs/thick spots with a utility knife blade, and lightly sanded the surface. Then I applied 2 more not thinned coats of epoxy, sanding between coats.
Thinning epoxy is normally not a good idea as it can weaken the epoxy, but for speaker building, it's not a problem. The penetrating ability of the thinned epoxy, especially on edges really stiffens the MDF. The subsequent coats of not thickened epoxy make for a nice dent resistant hard surface that helps prevent seam print through.
If your going to paint the cabinets, It's best to use some type of transition primer over the epoxy as some coatings have solvents that might attack the epoxy. I just use Zinsser Seal coat (shellac). Also, I wait 5-7 days for epoxy to fully cure before applying anything.
Epoxy has an extremely long shelf life. I have used 15 year old epoxy with no problem. The pumps do make life easier but I don't use them for accurate measuring, just for transferring to mixing cup. I prefer to use a digital gram scale to weight the epoxy/hardener. I can mix very tiny amounts using the scale. Most epoxy suppliers list their mixing ratios by volume and also by weight.