I'd strongly suggest that the StewMac filler is rebranded Aqua Coat.
It is relatively expensive (a lot cheaper in the US though) but a tub will do 5-8 guitars depending on size and depth of grain to fill. Which as long as you are doing that many guitars (or can use it on other woodworking projects) still makes it good value for money in overall terms. I know I'll spend a several times more per guitar on other consumables like nitro paint than on the grain filler. But it isn't worth it for a single guitar.