Many, many, moons ago I bought a 6x48 and tried to use it. It didn't really work for blades, but I still use it for a variety of grinding & shaping, especially for handle material. As for tips on plunge lines, are you looking for tips using the 3x36, or just tips on how to get them straight in general? If you are using mostly hand tools, you should probably get a file guide. You can buy one with carbide faces (kinda pricey) or you could make one from hardened precision ground flat stock. I made one and used it for years before I eventually got the one with the carbide faces. I still use the one I made for a variety of tasks and file work. You would clamp the blade in the jig with the edges of the jig where you want the plunge lines and start filing away the blade material using the jig as a guide.

Don't waste your money on that. Just save your money for 2x72. I know its not cheap but if you want to make Good quality hand made knives in a reasonable amount of time you need a 2x72. Those are really meant for sanding wood not grinding hardin HiCarbon steel.

Thanks guys! I'll definitely make a file guide and try it out. The 4x36 was given to me and I'm using it to just practice, practice, practice while I save up for a 2x72.

Well I started out with a 4x36 belt sander and it works ok if you use the idle drum like a grinder wheel and use zirconia belts you can hog off material fairly quick, and I learned how to make do. however, for a starter sander, i say get a sander of this style https://smile.amazon.com/gp/product/B01M68YKST/ref=oh_aui_search_asin_title?ie=UTF8&psc=1 .

the best thing about these are that the sanding belts act as the drive belt. with my old 4x36 the drive belt tended to wear out so i say get one of these to start and save for a bigger one later on.