I have a rectangular speaker cabinet (basically a box). Can I use spray lacquer and spray all sides at once? Otherwise I have to account for a lot of overspray.
Sure. You'll want something to rest one side on while it's drying. You can buy those triangle things like these: https://www.woodcraft.com/products/painters-pyramid-10-pack-new-improved?via=573621f469702d06760016d0%2C5764236869702d3baa0025aa or you could use some scrap plywood squares are drive a screw all the way through it. This way you you can finish the whole thing and let it dry without resting a side fully one a surface.